add configuration options for key color, key corner radius and key padding

Frieder Hannenheimrequested to merge
Frieder.Hannenheim/unfettered-keyboard:rounded_corners into main

Kind of fixes #8

With this pr it is much easier to tell which key a label belongs to. Key corner radius, key padding and key color can now be configured like this:

key_corner_radius: 15
key_padding: 4

key_color:
  r: 51
  g: 31
  b: 3

Screenshot from my desktop: image

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