Added an settings option to enable Developer
Mode, in which you can send UNO commands
from the Viewer. You can also add properties
to the command.

Change-Id: I76cbb55771cdd5cea89da65ddf67e48e12745b41
Reviewed-on: https://gerrit.libreoffice.org/37045
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>