Update SelectionTool.cs

Minor convenience. When Ctrl + Right-click is pressed, the selection is set to (or remains) 'Active'. With this, users can reselect their previous selection with the aforementioned input.

Removed unnecessary brackets.
Update SelectionTool.cs

When the user has already made a selection with the 'Selection Tool', they can now hold Shift, click and drag to modify their selection from the initial starting point.
Merge pull request #549 from SvardXI/patch-2

Update SelectionTool.cs