Cpaster frontend: Fix deployment

On OS X we may not set QMAKE_RPATH because otherwise the default rpath
to the Qt libs is removed.

Install the tool on Windows and Linux.
Add the plugins path to the rpath on Linux.
Also make the rpath when building relative instead of absolute on Linux.

Change-Id: Ieb6a79d9f9f922d117736ae7b4ae65c784ade5c9
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>