Refactor MathKeyboardComponent to reduce code size

Add styles for fixed right position
Customize markup for different positioning
Refactor Usage of KeyboardService in components

Setting on type button avoids that click events are evaluated from
the surrounding form

* Implement as draggable overlay
* Keyboard can be configured as 'mini' or 'full'

* Rename service to `KeyboardService`
* Replace the two fixed rows for upper and lower case letters
of the keyboard with a general row array
* Rename css classes, variables and methods