Allow inheritance

[fhennig]

Patterns should have a base pattern.

for this also the &white and *white need to be replaced. I could just use jinja again and allow to specify references to other properties like:

c:
  white: '#ffffff'
rofi:
  background: '{{c.white}}'

With this I could have also a definition such as border -> accent_colour and then define the accent_colour in a inherited theme. one where it is blue, one where it is green.