Adding colours to the feed to indicate type

[proycon]

This is a continuation on top of #218 (and includes it) , I'm adding support for colours to indicate toot types, but to maintain readability I needed to find an alternative selection method than setting a background colour, so I implemented one (opt-in) that makes the selected item bold and also underlines when on the focussed pane.

This still needs some fine-tuning (code cleanup etc). Screenshot of how it can look:

[proycon]

By default the colours are mostly unused and you have the regular behaviour, so to use this you have to opt-in with for example the following in your style section (or theme):

timeline-name-text=#808080                                                                                                                     
boost-text=#446672                                                                                                                             
media-text=#c630ce                                                                                                                             
card-text=#3a46a1 

# Alternative color-preserving highlighter that take effect instead of the background color                                                    
# default=0                                                                                                                                    
list-selected-boldunderline=1       

[RasmusLindroth]

Looking good! I'll look more at both your PRs and merge them later so you know :)