Blender Git Loki

Outliner: Rework element selection behavior

* LMB now replaces selection instead of adding to it. Shift+LMB adds to selection (or removes if already selected). This is usual selection behavior Blender.
* Outliner selection isn't completely separate from object/sequencer-strip/render-layer/... selection anymore, when selecting an outliner item we now always try to select (and activate) the object it belongs to. Previously you had to click the name or icon of an item to select the object (or whatever) and on empty space within the row to set outliner selection.
* Collapsed items may show click-able icons for their children (nothing new). Clicking on such an icon will also select the hidden item it represents now, you'll notice after opening the parent. This valid from a technical POV, I'm not sure if this is wanted from user POV though. Changing would be easy, feedback welcome!
* Code cleanup.

Part of T37430.