[snipmatch] Bug 447195: Share Snippet button

This adds a Share button to the context menu in the snippets view.
The button is is enabled when all selected items are snippets belonging to the
same repository which has to return true for the new
ISnippetRepository.isSharingSupported method.

This opens a EGit Commit Dialog, allowing the user to push (to gerrit).
Afterwards, a mixed reset is done on the local repo to get back to the
current fetch HEAD but to keep local (unstaged) copies of the users'
changed snippets.

Change-Id: I8aebcd4c695eab17469293890b7f23ca50d47049
Signed-off-by: Johannes Dorn <johannes.dorn@codetrails.com>