Blender Git Loki

Outliner: Selection cleanup

No functional changes. The outliner selection operators shared many
different functions for selection, activation, mode toggling, and other
actions, but the code paths were not always clear, making any changes
difficult.

This cleans up the code and uses outliner_item_select() as the base
function for selection, activation, mode toggling between the different
selection operators. It also prepares for future features and fixes.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D5817