viewer: Small code and comment cleanup
medialibrary: Fix the import progressbar to show translated text properly

Fixes bug #709537
undo: Undo redo logic for assets is now implemented in project.

https://bugzilla.gnome.org/show_bug.cgi?id=739251
undo: Disconnect from the signals correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=739251
mainwindow: improve signal disconnecting.

https://bugzilla.gnome.org/show_bug.cgi?id=739251
Start implementing scenario serialization

https://bugzilla.gnome.org/show_bug.cgi?id=739251
scenarios: add the type of the added asset

https://bugzilla.gnome.org/show_bug.cgi?id=739251
scenario: serialize action even if running

https://bugzilla.gnome.org/show_bug.cgi?id=739251
scenarios: serialize asset removed

https://bugzilla.gnome.org/show_bug.cgi?id=739251
scenarios: adding asset requires an id, not an uri

https://bugzilla.gnome.org/show_bug.cgi?id=739251
undo: disconnect correct handlers when layer is removed.

https://bugzilla.gnome.org/show_bug.cgi?id=739251
undo: No need for class attribute actions.

https://bugzilla.gnome.org/show_bug.cgi?id=739251
undo: Add LayerAdded and LayerRemoved actions.

https://bugzilla.gnome.org/show_bug.cgi?id=739251
scenarios: serialize layer addition and removal

https://bugzilla.gnome.org/show_bug.cgi?id=739251
scenarios: serialize clip addtion / removal

https://bugzilla.gnome.org/show_bug.cgi?id=739251
scenarios: The log file now belongs to the application itself.

https://bugzilla.gnome.org/show_bug.cgi?id=739251
scenarios: Serialize play / pause

https://bugzilla.gnome.org/show_bug.cgi?id=739251
previewers: hardcoded path manipulations are bad.

And I should feel bad.

https://bugzilla.gnome.org/show_bug.cgi?id=739251
scenarios: put scenario files in a cache directory.

https://bugzilla.gnome.org/show_bug.cgi?id=739251
utils/timeline: remove unused imports

https://bugzilla.gnome.org/show_bug.cgi?id=739251