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