Blender Git Loki

DopeSheet: Add menu entries and hotkeys to select all keyframes
left/right to current frame in dope sheet

This commit separates out this selection functionality out of the
click-selection operator into a separate operator, so that hotkeys and
menu entries can be assigned to it.

This is based on an idea+patch (#23738) submitted by Torsten Rupp
(rupp), though I've ultimately decided not to go with the suggested
implementation as I don't think this fits that well under the "column"
select operator.

Todo: Graph Editor support will be coming shortly...