Overhauled hacking chapter introduction

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Added aditional documentation on the backend implementation

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Added missing pieces to manual

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Added a custom widget example (digital clock)

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>