Add file list combobox to diff editor

Change-Id: I2a40207ed3c4a5c07ba544d681aed6649a0b1a11
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
C++: fix crash when failing to open non-existing file.

Change-Id: Ie7bd3b36139ab7fac1186cefa7222dff7277eb53
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>