This works to address T63996.
This makes it so that shift-clicking can select through the gizmo. This solves the biggest issue with clicking through gizmos, which is that they can get in the way when extending selections.
The tradeoff here, is that we then lose the ability to shift-click on gizmos to lock opposing axes, although we still have this via the new squares.
This patch only makes this tradeoff for left click select. Right click select is unaffected.