Allow ending wordwise select in mousetracking mode
Commit e2167eb0 restricted deactivating the word-wise/line-wise selection mode to mouse presses with no modifiers. Since in mouse-tracking mode, the Shift key has to be pressed to not pass mouse events to the application running on the terminal, there was no way to end word-wise/line-wise selection mode without sending a mouse event to the application.
Modify the logic so ending word-wise/line-wise selection mode is done by a LMB button press with no modifiers on non-mousetracking mode, and by a LMB button press with only the Shift modifier on mousetracking mode.