CustomWizard: s/struct/class/

Consistently use class to avoid warnings about class/struct mismatches.

Change-Id: I1ebd7b5b4687ed93db7f66c37193c9ca3900dede
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Qmake project: HEADERS are not sources

Fine tune the extra compilers support to know that we already show
headers in a separate category. HEADERS is the input to the moc
extra compiler.

Task-number: QTCREATORBUG-11459
Change-Id: I60e9c6b9eb2d55b7a0b02b5f63280b6268c42704
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>