GUI thread freezes while indenting file

1. Open the attached qml-file.
2. Select all the characters in the file.
3. Press Ctrl+i to indent the file correctly.
   This will take a while. While Creator is busy, the GUI thread is frozen.

The GUI should remain responsive while the editor is busy. It might instead display a spinning wheel like the git-editors do when waiting for content.

Found by Squish test tst_qml_indent.