timeline: Do not seek for operations such as sliding (moving) clips
viewer: Disable GTK double buffering to prevent flickering and black images

Fixes bug #708088 (viewer image sometimes disappears/becomes black)
Fixes bug #596954 (flickering and black image while resizing the previewer)

Also hide the internal ViewerWidget at startup until a pipeline is set,
to prevent the user's wallpaper from showing up through the viewer.
viewer: set ViewerWidget's "sink" var on pipeline change

This variable was pretty much never set to anything other than None,
and we were somehow relying on it not being None...
viewer: Use the "realize" GTK callback to asynchronously obtain window handle

This eliminates some race conditions resulting from the use of GstVideoOverlay.
Namely, this allows docking the viewer correctly if it was undocked on startup.

Note the new style of getting win32 handle.
timeline: Remove dead code
mediafilespreviewer: Remove sync-message handler and add proper xid handling

Sync message handlers tend to slow down Pitivi, and the xid is no longer saved
in the preview widget as this was racy. Eventually, we should refactor this
so that all the handling of GstVideoOverlay is done in only one place.
Updated Slovenian translation
Updated Lithuanian translation
transitions: Prevent infobar from being stuck in the "shown" state with GTK 3.10
dogtail tests: Add a test for cancelling a project load
mainwindow: Handle cancelling choosing a project to load with no fallback

This fixes the scenario where you have no unsaved changes, Ctrl+O to choose
a project to load, then cancel the file chooser and then try to import clips;
it would fail because we didn't properly reset to a blank project instance.
bin: Fix a little screw up in Gst version checking
bin: Build devtools only if in development
clipproperties: Prevent the effect toolbar from showing when reselecting a clip
Kill ChangeLog with fire

This is not 1999. We have working version control now.
i18n: Clarify the "Contributors" paragraph in the about dialog for translators

Fixes bug #709003
pixmaps: Fix the installation of effects and transitions icons

This ensures they install into subdirectories of the pixmaps directory,
otherwise Pitivi wouldn't find them when installed system-wide.
Releasing Pitivi 0.91 "Charming Defects"
Updated Brazilian Portuguese translation
Updated Spanish translation