Fixed: menus not closing on left click.

Previously, clicking outside a menu on a control would not close the menu. Also clicking on the menu header again would open a second menu.

This error was caused by the control receiving the click and starting its own tracking, depriving the menu of the mouse up event it was awaiting.

The behaviour to wait for the mouse up was wrong to begin with. Menus should close on mouse down.

This fix closes the menu on left mouse down anywhere but the menu, while still allowing the click to pass through to any control underneath the cursor.

Fixes #1833.