Highlight Usages in File (Ctrl-Shift-F7) now toggleable
Press Ctrl-Shift-F7 on 'a' highlights all usages of variable 'a' in the file.
Press Ctrl-Shift-F7 on 'b' adds all usages of variable 'a' to highlights.
Press Ctrl-Shift-F7 on 'a' again removes highlights of variable 'a' usages.