(trunk, qt) #5723: 'Simplify file tree icons drawing' -- patch by mike.dld

git-svn-id: file:///data/workdir/Transmission-svn//trunk@14315 1bcf1a7e-6170-4b50-b784-84ec52b2dec6
Use built-in _WIN32 macro instead of WIN32

git-svn-id: file:///data/workdir/Transmission-svn//trunk@14316 1bcf1a7e-6170-4b50-b784-84ec52b2dec6