Initial modifications: adjusted behavior, improved robustness, cleanup

Tablet mode no longer rotates the screen. Stylus monitoring is by
default not activated, since Ubuntu handles palm rejection just fine
now.

In the previous versions, multiple clicks on a "start monitoring" button
would start a new process for every click and leave the previously
started ones running wihtout identifiers, with no mechanism to terminate
them. This is fixed by initially creating the processes in the
constructor, and checking their state before starting them when the
relevant button is clicked. When the stop button is clicked for a
running process, the relevant process is terminated and a new process
with the same identifier is created (but not started).

Tabs replaced with spaces; line lengths and docstrings edited to
conform with style guidelines; some variables and methods renamed for
consistency; GUI creation extracted into own method.

Added my name to copyright statement.
Changes in user interface

Renamed "display position monitoring" to "device state monitoring", and
renamed all variables, methods and messages accordingly. Renamed the
orientation changes, from "left" to "orientation left" etc., and renamed
their messages accordingly. Reordered buttons in the GUI.
Re-enabled stylus proximity monitoring by default

Turns out, there is no palm rejection in Ubuntu after all. Stylus
proximity monitoring is therefore re-enabled by default, and the README
edited accordingly.
Changed order of buttons in the GUI

Laptop mode before tablet mode, and normal and inverted orientation
before left and right orientation, feels more natural. The order of
methods in the code were changed according to a similar logic.