Holding control while dragging text to copy it was being blocked by link
handling stopping the propogation of the mouse event; solution: don't
handle link clicks when selection_drag_state is Dragging.

Issue found & fix tested on Linux (since the modifier key on mac is
apparently different, the issue likely doesn't occur there, but I'm
assuming it does apply to Windows).

I didn't see an issue open for it, and since I fixed it myself I'm
unsure whether it needs one or if the PR is enough.

Release Notes:

- Fix an issue where Ctrl+drag to duplicate selected text on Linux and
Windows when the selection is inside a link.