ui: qemu: make DisplayEdit declarative and drop DisplaySelector

ui: qemu: make DisplayEdit declarative and drop DisplaySelector

DisplayEdit did not get a cleanup during our ExtJS 4 to ExtJS 6
cleanup, in fact it was really touched in the last 5 years.

So refactor it to a declarative syntax, at least almost, we keep the
me.load in initComponent, just to much hassle to adress this for now.

Also purge DisplaySelector. It was a component inheriting from
KVComboBox with the possible VGA values as items only used here.
So just use a KVComboBox directly and set comboboxItems directly.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>