Use correct paths for assests on OS X

Signed-off-by: Rob Caelers <rob.caelers@gmail.com>
SizeGroup now support horizontal + vertical simultaneously

Signed-off-by: Rob Caelers <rob.caelers@gmail.com>
Reduce code sharing between gtk and qt5 for the time being.

Signed-off-by: Rob Caelers <rob.caelers@gmail.com>