Merge lp://qastaging/~3v1n0/unity/panel-p-cleanup into lp://qastaging/unity

A lot of panel fixes with code cleanup and improvements to the panel code.

This branch introduces a lot of code cleanup and improvements to the panel code.

A part the linked bugs fixed, this changes:
 - PanelStyle: used now to check all the panel related settings, including
   fonts and DPI. Emitting the changed signal when they get changed (so now
   when changing the title font, it gets updated immediately).
   Added better support for HighContrast themes and fallback buttons.
   Used ti everywhere to remove the "24" height magic number.

 - PanelIndicatorEntryView: code cleaned a lot to be more C++ conformant,
   and modified to be easily extendible in the near future. Also fixed issues
   with the indicator entries and Scale/Expo

 - PanelMenuView: the layout system has been rewritten and now it works
   natively without the workarounds we were using before,
   fixed the panel paddings to match design specs;
   Optimized the drawing operations, now the title texture is cached and
   rebuilt only if really needed.
   Fixed the panel title on Expo and Scale.
   Improved the detection of the panel used to draw the app title on
   multi-monitor (disabled otherwise), and the alt+tab/alt conflict.
   Maximized windows list is re-populated on startup or when adding a new
   screen.

 - PanelTitlebarGrabArea: rewritten to better handle the grabbing of the panel
   and clicks over it.

 - WindowButtons: rewritten using in a better way the subclass nux::Button
   features, factorizing some code for each button into WindowButtons. Also
   the handling of the controlled window is now done internally, not by the
   PanelMenuView.

A lot of panel fixes... Into one branch, because they should go together, but the single changes are shown on these single code reviews:

1) PanelStyle changes: http://go.3v1n0.net/Hs4YM6 [Tim: Approved]
2) Indicator Entry changes: http://go.3v1n0.net/Hp8VDp [Tim: Approved]
3) Indicators View changes: http://go.3v1n0.net/GXHKN3 [Tim: Approved]
4) Window buttons changes: http://go.3v1n0.net/GZHigm [Tim: Approved]
5) Grab area changes: http://go.3v1n0.net/H81Syb [Tim: Approved]
6) Tray changes: http://go.3v1n0.net/H6yqba [Tim: Approved]
7) Window Manager changes: http://go.3v1n0.net/H2GDwr [Tim: Approved]
8) Core changes: http://go.3v1n0.net/H1qokF [Tim: Approved]
9) Menu view changes: http://go.3v1n0.net/HwATeM [Tim: Approved]
10) Panel view changes: http://go.3v1n0.net/H8lwdB [Tim: Approved]

This branch introduces a lot of code cleanup and improvements to the panel code.

A part the linked bugs fixed, this changes:
 - PanelStyle: used now to check all the panel related settings, including
   fonts and DPI. Emitting the changed signal when they get changed (so now
   when changing the title font, it gets updated immediately).
   Added better support for HighContrast themes and fallback buttons.
   Used ti everywhere to remove the "24" height magic number.

 - PanelIndicatorEntryView: code cleaned a lot to be more C++ conformant,
   and modified to be easily extendible in the near future. Also fixed issues
   with the indicator entries and Scale/Expo

 - PanelMenuView: the layout system has been rewritten and now it works
   natively without the workarounds we were using before,
   fixed the panel paddings to match design specs;
   Optimized the drawing operations, now the title texture is cached and
   rebuilt only if really needed.
   Fixed the panel title on Expo and Scale.
   Improved the detection of the panel used to draw the app title on
   multi-monitor (disabled otherwise), and the alt+tab/alt conflict.
   Maximized windows list is re-populated on startup or when adding a new
   screen.

 - PanelTitlebarGrabArea: rewritten to better handle the grabbing of the panel
   and clicks over it.

 - WindowButtons: rewritten using in a better way the subclass nux::Button
   features, factorizing some code for each button into WindowButtons. Also
   the handling of the controlled window is now done internally, not by the
   PanelMenuView.

Testing:
 - All the panel items now are introspectable. And this allow AP testing.
 - The branch with all the brand new tests for the panel are at:
   lp:~3v1n0/unity/panel-p-tests