Increase selection

In selection mode, when at least one area is already selected if drawing a new selection (dashed green rect) while META (Ctrl or Cmd) modifier is pressed, then do no replace existing selection by new one, but add new selection to existing one (areas from first and second selections are both selected, part of merged selection).

If there is no previous selection, modifier can be ignored.

**Start event** is either:
- `mousedown`, if META is already pressed (before starting to draw selection).
- `keypressed` with META, if has previously received `mousedown` (start drawing selection) and `mouseup` hasn't been yet fired (to stop drawing selection).

It means that user can press & release META modifier several time until `mouseup`, each time we choose switch between plain selection tool (which will be indicated visually by #173) and update selection tool (which will be indicated visually by #174 ).