Restructure: components/ -> imports/QtDesktop/

Makes it possible to pass the local imports tree to qmlscene:
$ qmlscene -I imports/ examples/Gallery.qml

Later on, auto tests can run against the local imports
tree instead of having to depend on the install step.

Change-Id: I1874a5862e8c54463473843befd09097e8563e9c
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@nokia.com>