widget: Disable more widget keybindings

Disable all of GtkWidget's keybindings except the ones for popup-menu.
We do not want that either, but handling Shift-F10/MenuKey ourself will
require new API to en/disable it, so defer that for now.

This reverts commit 46b7464576141d69bd0dce16409141cfb26a8225, wontfixing
bug https://bugzilla.gnome.org/show_bug.cgi?id=153265.

https://bugzilla.gnome.org/show_bug.cgi?id=726438
build: Remove unused pty.h header

https://bugzilla.gnome.org/show_bug.cgi?id=727540
widget: Initialize the color palette when the widget is initialized

https://bugzilla.gnome.org/show_bug.cgi?id=728051
widget: Allow passing NULL colour

These were already (allow-none) but neverthelss asserted the colour != null.
widget: Fix font-scale property
vteapp: Reimplement vteapp in vala
widget: Make resize-window signal cell based

And don't report the padding down to the application.

https://bugzilla.gnome.org/show_bug.cgi?id=555662
vteapp: Fix a couple typos
vteapp: Warn when parsing the colour failed
widget: Use gsize/gssize in public API
widget: Add function to check for match at event coordinates

This saves the caller from translating from event coordinates to
grid coordinates himself.
build: Fix vteapp build dependencies and includes
Updated POTFILES.in