Initiate a move drag immediately upon press to a window handle

With the standard mutter server-side decorations (and the client-side decorations in Chromium), a press (mouse down) directly on a titlebar will start a move drag immediately.

Mutter has a heuristic to raise the window if a move drag doesn't travel far. This is important for supporting no-raise-on-click, a classic window manager behavior.

With this change, Mutter's heuristic will now work properly without any special logic hardcoded in Gtk and client decorated windows will behave more consistently compared to server decorated windows.

The existing behavior of interacting with widgets in the window handle is retained: gtk-double-click-distance is still read to avoid initiating a move when not desired, and widgets that handle "pressed" already will continue to do so.

Closes #2710