Merge lp://qastaging/~mardy/unity-2d/treat-double-clicks-as-single into lp://qastaging/unity-2d/3.0

[panel & launcher] Consider double clicks as single clicks for launcher items and the home button of the panel (bfb).

For the panel created HomeButton class: specialized subclass of QToolButton, handling double clicks as single clicks.
For the launcher, ignored DoubleClicked signal and added a workaround that separates click resulting from the end of a drag and drop from regular clicks.

Proposed fix for bug 766776.

For the BFB, use a QTime instance to track the elapsed time since the last click. I initially tried hacking around a reimplementation of the virtual mouseDoubleClickEvent(), just to find out that it's never emitted because QAbstractButton is reimplementing the handling of mouse events in order to suppress it.

For the launcher, I wasted some time in figuring out that the onClicked() slot in the MouseArea in LauncherItem.qml is never emitted (should we add a comment next to it, to avoid someone else to spend time in figuring out that as well?): this is because the LauncherItem is part of a bigger MouseArea (the one in ListViewDragAndDrop.qml) which eats all the mouse events.
I initially tried to set the mouse.accepted property to false in the outer MouseArea, and I could fix the double-click problem in LauncherItem very nicely. But this broke the DnD, because after setting the mouse.accepted property to false, the ListViewDragAndDrop would not be notified for the next mouse move and release events.
So, though quite ugly, the ignoreNextClick flag is the only workaround I could think of. QtQuick 2.0 will probably help in solving this better [http://bugreports.qt.nokia.com/browse/QTBUG-13007].