timeline: commit to the action log on drag and drop.

https://bugzilla.gnome.org/show_bug.cgi?id=741668
titleeditor: Make sure to set default values and update widget afterward

And make default background black, because of: https://bugzilla.gnome.org/show_bug.cgi?id=681447

https://bugzilla.gnome.org/show_bug.cgi?id=741670
ruler: Reuse height value
ruler: Add top margin

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=738481
clipproperties: Remove obsolete function
Use Gtk.Box instead of Gtk.VBox because Gtk.VBox is obsolete
elements: Fix padding argument for pack_start
Use Gtk.Box instead of Gtk.HBox because Gtk.HBox is obsolete
Fix spacing setting
clipproperties: Add the vbox directly to the ScrolledWindow

We depend on Gtk 3.10 and this is available since Gtk 3.8.
Set fill=False when expand=False

The fill parameter has no effect if expand is False.
This should clarify a bit the code.
mainwindow: Fix the save-as call when closing an unsaved project
mainwindow: Pass correctly the uri of the project which failed loading
project: Cleanup the logic tracking whether all the assets of a project loaded

This actually fixes a bug: When an asset cannot be found and no
replacement is provided, the _projectManagerMissingUriCb method was
disconnecting itself as the missing-uri signal handler and then
_destroyedCb tried the same and failed.
Add an etched-in border around the title editor and clip properties

So they are consistent with the Transitions and Media Library panes.
timeline: Reuse the same zoom fit logic
timeline: Zoom-fit when rendering, to avoid generating a lot of thumbnails