Add base for GUI testing

Rework of the now stale !1018 (which can be closed by the way) to fix broken tests.

The goal is to add GUI testing using appium and the AT-SPI selenium webdriver. While the current tests are not exhaustive they still test basic functionalities and provide a base that can be expanded later with more tests.

Some of the tests had to be disabled due to unstable behavior. I am unable to tell if the root of the problem is with Okular or the AT-SPI interfacing with the selenium driver. Still left them in in case anyone wants to take a look later.