Main issue in previous code was that you could not shift-alt-rmb select several
contiguous strips, result was pretty much unusable.
Also, enhanced general behavior of this selection mode, now (similar to alt-rm clicking
on handles), when you alt-rmb click on a same strip several times, you alternate between:
* Strip is selected, neighbor handles are selected;
* Strip and its handles are selected, neighbor handles are selected.
…which allows you to either grab or slide the strip.
And refactored a bit code too, linked_handle has a complete different logic
than without this option, simpler and clearer to completely separate them in code.
Initial issue reported by Leon Cheung on IRC, thanks!
f5629b7265