rework actionbar and OnBackPressedListener, cleanups

remove a lot of code duplication
speed up switching animations, decrease it from 500ms to 300ms

Change-Id: Ic3bd350328ffa5a92e2bd4edc0b274cce12466f5
rework fragment loading, remove duplicate code, more cleanups

Change-Id: I42a2e80bc2d2d3101962ce1793a2bd570c37cec5
increase animation time back to 500ms

Change-Id: Iff6f257f8c4a0bae8fd3e1cc1c049edc34623cb9
allow to toggle between a dark and light theme

Change-Id: I75bc0166216ff1be6d2b0cdb462c695336ecea1c