clipproperties: Work around bugs in the storemodel data setting code

We now simply store the selected effect directly on the storemodel and
use it so we avoid the use of pickle and other 'complicated' workarounds

Work around https://bugzilla.gnome.org/show_bug.cgi?id=737587

Conflicts:
	pitivi/clipproperties.py
Updated Polish translation
help: updated screenshot for the Main Window

This one needs no translating as it uses numbers instead of labels.
help: Mention "menu button" rather than "app menu"

"App menu" could be confused with the global menu that you find in GNOME Shell
help: Be realistic about expectations of supporting legacy capture technology
help: Remove the GTK+ FileChooser screenshot

It's not extremely important, and the FileChooser changes often these days
help: Remove the "hovering tooltips" screenshot

The screenshot is very old, and tooltips are kind of self-explanatory.
timeline: Fix dragging of second clip from the media library to the timeline

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=737634
timeline: Scroll to playhead when the zoom changes

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734064
Revert "gdk: fix icon too large warning"

The warning will be removed in Gtk.
Fixes https://bugzilla.gnome.org/show_bug.cgi?id=737499

This reverts commit 409d4773d52b0080a1d94e0c62a7acc8619098ed.
timeline: Clarify what's being used in _scrollEventCb
build: fix make install

Since commit 0de1e04, signal.py no longer exists. So let's also not
attempt to install it in the end.

https://bugzilla.gnome.org/show_bug.cgi?id=738324
Set our Gst dependency to 1.4.0

We actually depend on 1.4.0, not 1.4.3

https://bugzilla.gnome.org/show_bug.cgi?id=738378
GNL is dead, do not check if it is present!
undo: Undo redo logic for assets is now implemented in project.
undo: Disconnect from the signals correctly.
mainwindow: improve signal disconnecting.
WIP for scenarios
scenarios: add the type of the added asset
scenario: serialize action even if running