Remove Qt5Main stuff, link to new imported target

Change-Id: Ibd6b46c1ba2c1738307f559aafd2af75a1af8aaa
Modifications needed when using Qt 5

The increased use of CMAKE_PREFIX_PATH complicates things, need to
be more careful to actually iterate rather than assume it is a
single value. Need to install the Windows integration plugin otherwise
nothing will work on Windows.

Change-Id: I8318d49e8b0c48af7e18e91baf2f90f0c379aa5c
Merge branch 'cmake-qt5-packaging'

Change-Id: I471bd7c747e62927ecf75ee890fd4b88b1863850