FileStore: make editable to allow filtering

FileStore: make editable to allow filtering

Making this ComboGrid based component editable allows implicit
filtering the backing store.
This is quite helpful on file selection, as for example the ISO file
selector from the create wizard is.
Clear the filter (and its displayed value) on store selection change,
as there the filter becomes void - at least this is a simple and easy
solution without glitches.

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