Added support for double/triple click+dragging.

Now double-click+dragging automatically snaps both ends to word boundaries
(unless on series of spaces), and triple-click selects whole lines.
As a side effect, snapping now occurs on button press, not button release
like it previously was, but I hope that won't be inconvenient for anyone.

Signed-off-by: Christoph Lohmann <20h@r-36.net>
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Gitea Version: 1.28.0+dev-505-g31b4d79a84