webos_qmake5: Add WEBOS_INSTALL_BINS Qt variable

Applications can use this variable to set their target.path when
installing generic binaries.

Open-webOS-DCO-1.0-Signed-off-by: Mikko Levonmaa <mikko.levonmaa@palm.com>

Change-Id: I76ad7de8cb4f33e9920fb384090a43e38b537b1f
Reviewed-on: https://g2g.palm.com/1731
Reviewed-by: Mikko Levonmaa <mikko.levonmaa@palm.com>
Tested-by: Mikko Levonmaa <mikko.levonmaa@palm.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>