Added active index which is updated on keyboard arrow up and arrow down events
When select is in focus and arrow up or down is pressed the option list will be shown
Visualizing the focused state of the component:

- When in focus a blue outline is shown around the input field of the select
- Fixes bug: when an option was selected by clicking on it the component would loose focus
- When hovering a mouse over an option it becomes "active" just like when
using the Down and Up arrows on the keyboard => seamless integration with both mouse and keyboard
- CSS animation rules for the hover state now are not needed:
light selection in the option list does not mean "hover" now, but rather "active"
Possible to select options with keyboard and Enter