Blender Git Loki

Outliner: Fix selection extend not toggling

An unintentional side-effect of rBfe7528ee919b was that when
extend-selecting a selected element in the outliner, it would be
deselected and activated rather than selected and activated.

This commit restores the expected toggling behavior. Consistent behavior
for extend-selecting child datablocks is not resolvable without a much
larger cleanup of the outliner select functions.