titleeditor: Simplify the UI

Stripped the buggy UI so that the text field shows only the text,
without colors etc. The font and color buttons now take effect on the
entire text, not only on the selection.

Removed the bold and italic buttons as these are specified by the font
chooser button.

Fixes:
https://bugzilla.gnome.org/show_bug.cgi?id=726473
https://bugzilla.gnome.org/show_bug.cgi?id=708412
titleeditor: Add default foreground and background colors
Use GObject.Object instead of Signallable
mainwindow: Avoid using app to access the current project
pipeline: Remove unuseful 'format' parameters and fields
pipeline: Remove duplicate check of position value
dogtail: Update dogtail tests to run on latest Gtk
dogtail: Fix tests depending on click items in the main menu
dogtail: Integrate big helper method in test method
dogtail: Cleanup test_effects
mainwindow: Cleanup the method for creating the stock icons
mainwindow: Show a title when the startup wizard is visible
timeline: Get rid of some clip animations

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=719348
mainwindow: Remove a vbox element which is not useful
mainwindow: Consistent imports and move dead code around
clipproperties: Fix effect configuration UI destruction

The second resetKeyframeToggleButtons call was raising an exception.
previewers: Fix initial value of AudioPreviewer.lastUpdate
video previewer: remove unused self._callback_id
previewers: Always set lastUpdate when computing geometry
previewers: Avoid removing sources already running

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726904