Improve SHIFT handling of Selection Manager

[Alecaddd]

Summary / How this PR fixes the problem?

Make the SHIFT modifier behave more consistently.

This PR fixes/implements the following bugs/features:

• Hold SHIFT and click on a selected item => Remove the item from the selection.
• Hold SHIFT and click + drag on an empty area => Doesn't reset the selection, allowing to area select more items.
• Items that are already part of the current selection are not highlighted when inside the bounds of a new selection area.