Review Preferences dialog description

Add description of RGB and CMYK color profiles.
Add TODO comments to be done after 2.8.2 release.

German translation:
* Update screenshots.
* Synchronize screenshots and text.
* Fix spelling errors and offhand writing.
* Update Preferences/Help (German translation):
    If the internal GIMP help browser is not installed,
    then the next 2.8 release will hide the listbox
    'Help Browser to use' and always open the help files
    in the system's standard web browser.
    Remove obsolete line 'Linux screenshot' because it
    doesn't provide useful information in this context
    and lets assume that this feature only works on
    Linux, which is not the case.
    Some minor fixes.
German translation: fix save/export explanations

Rename some untouched 'Save' references to 'Export' to clarify
the new paradigma in GIMP.
Some fixes in German translation

Fixes in Text tool (screenshot, text).
Update screenshot of Brush size widget.