dt2w: Add an option to specify the dt2w touch area

The previous option doubletap_full_window is replaced with a more
generic option: "doubletap_area" can have 3 values:
- 0 (default): no check on touch area (i.e. full screen);
- 1: bottom area (left/right margins excluded);
- 2: top area (left/right margins excluded);
- 3: center of the screen.

Signed-off-by: Mikael Berthe <mikael@lilotux.net>