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.