Redo csd window-dragging
The window-dragging code had a number of issues: The code was starting a drag on every button press, never bothering to cancel them. This leads to the odd hand cursor occurring between the two clicks to maximize. We relied on GDK's multi-click detection, which gives us triple-clicks when we really want sequences of double-clicks. Lastly, we didn't propery restrict double-click handling to the primary button, so e.g. if you had a window on an empty workspace, double-right click on the titlebar would maximize it, which is not intended. This commit solves all three problem by a doing our own double-click detection, and only starting a drag when the pointer goes out of 'double-click range'. We change the way dragging is implemented for menubars and toolbars to just letting events bubble up, so they get the same behaviour as the titlebar. To make this work, we have to select for pointer motion events in a few more places.
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