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
ruler: Highlight only important timestamp elements