LinGui: add a hidden feature to allow tweaking some settings. currently

LinGui: add a hidden feature to allow tweaking some settings.  currently
tweaks are available for detelecine, decomb, deinterlace, deblock, and denoise
to enable, you must manually edit preferences file and add "allow_tweaks=1"
to tweak a setting, right click on the widget, an entry dialog will appear.
minimal validity checking is done on the tweak's value.

git-svn-id: svn://localhost/HandBrake/trunk@1648 b64f7644-9d1e-0410-96f1-a4d463321fa5