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