public API for base widget state
renamed base widget state pressed -> clicked. this is more appropriate, since it's about a basic mouse action. pressed is more appropriate for the state of a button.
no need to override methods from Slider, since VSlider doesn't add any new methods
add accessor methods for special widget state
add text() getter method
added API docs as own file so they can be updated while not touching README (which also contains a download link to the last release)
added auto connect to all widgets