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titleeditor: Fix the background color button

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=708411
titleeditor: Fix the background color button initialization
ui: Remove two unused methods
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