Makefile: code clean.
Add pressure sensitivity to show the dash.

Add an option to use the pressure barriers and a pressure threshold to show the
dash when available. This also allow to use the autohide in mutltimonitor
configurations with the primary monitor on right. The option is enabled by
default.
Bug: put to zero a timeoutid after executing it.

Avoid warning concerning the removal of an non existig timeout. I think in the
past there wasn't this warning and the remove was just ignored.