Added possibility to extend dusk browser object.
Added possibility to run tests in phone/tablet/desktop mode.
TODO: Fix tests in phone and tablet mode
I trust this is a non-intrusive change, but ffs, zero-regression;
While there's no refined definition for desktop UX, let alone phone/tablet, the latter few are not the focal points of our efforts to date I hope.
Yes, I just copied some code I already have had in Roundcube for future use. The main point was to have possibility to extend the browser object with some helper code, to simplify tests writing.