Disable the GTK pixel cache due to rendering artifacts

This is a hack, but fixes the issue. When we have a GTK+ fix we can remove this.

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=726475
updates: use a valid icon size for symbolic icons

0 means GTK_ICON_SIZE_INVALID, which produces the wrong result on HiDpi
displays.
Do not show 'Installed' on the app tile when actually installing

Also, not a new string so no need to worry.

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=727573
trivial: Fix a critical warning introduced in the last few patches
Release version 3.12.1