Make sure we load translations on the main thread.

Because the old implementation (accidentally) worked when you put the
ecm_create_qm_loader call in a different CMakeLists.txt file to the
target the file was added to, some projects did this.

This won't work with build-time-generated files, though, like moc files.
So we (ab)use QTimer events to make the loading happen on the main
thread.

BUG: 346188
REVIEW: 126000