transitions: don't activate when audio transition.

+ remove unused imports
render: Don't show the details of the errors raised by GStreamer pipelines

They're useless for users as they're a string of the path/hierarchy of objects,
and they'll never be enough information to troubleshoot the issue by themselves.

Fixes bug #739832
docs/release: Correct the tarball download URLs
pitivi-git-environment: Fix the comment on how to run "master"

Also remove redundant code that was handled elsewhere
Updated Polish translation
GNONLIN IS DEAD

Long live GES & NLE!
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Use Gtk.Box instead of Gtk.VBox because Gtk.VBox is obsolete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Use Gtk.Box instead of Gtk.HBox because Gtk.HBox is obsolete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.