[repoView] Add "Open With" menu for workspace files

Note Eclipse UI's OpenWithMenu class only works with IFile, so it's only
shown for files that are in the workspace.

Bug: 444457
Change-Id: I262637fcfe70b5173bf1a18515c7100d91dc6c1d
Signed-off-by: Robin Stocker <robin@nibor.org>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
Merge "[repoView] Add "Open With" menu for workspace files"