Add filtering with help of DirCacheCheckout.getContent()

Add filtering with help of DirCacheCheckout.getContent()

see: https://git-scm.com/docs/git-mergetool

* refactoring of content (FileElement) handling
* now the temporary files are already filled with filtered content in
the calling classes (PGM), that can be used with EGit content too

TODO:
 * keep the temporaries when no change detected and the user answers no
to the question if the merge was successful

Bug: 356832
Change-Id: I86a0a052d059957d4d152c1bb94c262902c377d2
Signed-off-by: Andre Bossert <andre.bossert@siemens.com>