vteapp: Use gtk API to get the text targets

Conflicts:
	src/vteapp.c
vteapp: Add option to add new dingus

For testing purposes, allow specifying extra dingu regexes
on the command line.

Conflicts:
	src/vteapp.c
vteapp: Derive from VteTerminal
vteapp: Correct error message
vteapp: Remove dead code
widget: Init variable

Fixes a potential condition depending on an uninitialised value.
widget: Add font-scale property

To implement zoom, add font-scale property.

Conflicts:
	src/vte-private.h
	src/vte.c
	src/vteapp.c
Revert "widget: Install default CSS at FALLBACK priority"

This causes a off-by-one error somewhere that makes sizing not work correctly
anymore. So revert this for now.

This reverts commit 8d970eaf129221d925a26f4423b4baf4524ff481.