Keyboard modifiers should work with selection highlight box

[hudson-newey]

The selection highlight box does not follow our current keyboard modifier behaviors of:

• Ctrl to add to the current selection

We should make the highlight selection box have the same keyboard modifiers as the current selection type.

[atruskie]

• Ctrl + drag toggle selection in the selection rectangle
• Shift + drag is always an additive addition of the selection rectangle to the sub-selection
• Ctrl + Shift + drag is the same as Ctrl + drag