* add TimetableController, this class will handle all timetable work (main + additional Subjects)

* minor style fix: nav_header_main set background to black and text color to white

* initially select the current theme in the select theme dialog
* set the correct accent color for all dialogs (needs to be done manually for every dialog)

* gradle 6.2.1 -> 6.4
* constraintlayout 2.0.0-beta4 -> 2.0.0-beta6
* coroutines 1.3.5 -> 1.3.6

* commons-lang 3.9 -> 3.10
* coroutines 1.3.3 -> 1.3.5

* kotlin 1.3.70 -> 1.3.71
* gradle-plugin 3.6.1 -> 3.6.2

* update kotlin 1.3.61 -> 1.3.70

* call the onboarding activity on first start
* reworked selectCourse(() to work outside of the SettingsFragment
* closes #36

this Commit adds a Onboarding Activity, it is currently not usefull or fully working, see #36

* constraintlayout 2.0.0-beta3 -> 2.0.0-beta4
* material-components-android 1.0.0 -> 1.1.0
* this is the first release candidate for version 0.5.1

* added a Mensa shortcut, to directly display the Mensa-Screen
* kotlin 1.3.60 -> 1.3.61

* kotlin 1.3.50 -> 1.3.60
* androidx.constraintlayout 2.0.0-beta2 -> 2.0.0-beta3

* don't debug timetable cod on Sundays, since there are no lessons on Sunday

* minor GUI improvements

* cleaned up some timetable gui code
* close #33

* appcompat 1.0.2 -> 1.1.0
* material-dialogs 3.1.0 -> 3.1.1

* updated gradle wrapper to gradle 5.6.2