Allow to edit filename of staged images

By double-click on the filename it turns into an edit box that allows
to change the filename. This is e.g. useful if an image is pasted and
a filename should be assigned.

Change-Id: Iceb76d439945e26d9185bb2654626c30b79b7beb
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>