Handle forward/backward mouse buttons on press on Linux

The forward/backward buttons should be handled on press instead of release. This behavior is observed in both Firefox and Chromium on Linux (though on Windows it should stay on release).

In addition, there is a bug on Linux that prevents the mouse release event from being delivered when the mouse moved slightly. This makes the current behavior of handling these buttons on release to be a bit hard to use, since it requires a steady hand.

Attached is the patch that was applied to Assistant integration in Qt Creator.