Merge lp://qastaging/~3v1n0/bamf/desktop-class-matching-improved into lp://qastaging/bamf/0.4

BamfLegacyWindow and BamfMatcher changes needed to get the proper WM_CLASS / desktop-class matching in BAMF.

BamfLegacyWindow and BamfMatcher changes needed to get the proper WM_CLASS matching in BAMF.

This mostly allow to get the wanted experience with the Chrome/Chromium web applications
so now:
 - When a Web application is launched and has a matching .desktop file (this means that
   the file should include the StartupWMClass parameter that matches the window's class
   instance name) then it's matched as a new application using that file.
 - When a Web application is launched and has not a matching .desktop file (no file
   matches its class), then we fallback to the know .desktop files (so very likely to the
   parent application desktop).

Changes include:
 - BamfLegacyWindow is now able to fetch both the window class name and instance name also
   for non-gtk3 builds (instead of using libwnck, we fallback to Xlib)
 - BamfXutils now avoid to open a display connection per request.
 - BamfMatcher includes now the utility function
   bamf_matcher_get_application_by_desktop_file that allows to remove a lot of code
   duplication. It correctly matches the chromium web applications and avoids the re-usage
   of non-optimum running .desktop files.