Extend GuiComponents to work with keyboard / controller input

[gurkenlabs]

Currently, it is only possible to properly use most of the GuiComponent features with mouse input. There are several cases, where keyboard or controller input should be supported.

This will require to keep global track of which GuiComponent is currently focused for editing. This is already done for the TextFieldComponent in some way.
The support for these input devices includes:

• Menu : Select elements via UP/DOWN binding
• Slider: move slider left to right or top to bottom
• CheckBox: Trigger
• ListField: Select Element
• NumberAdjuster: increase/descrease number

[steffen-wilke]

This is related to #31