Introduced more (configurable) colours in chat messages to better distinguish different message types

[proycon]

Makes various colours configurable and allows sets different colours for different kind of messages like:

• media
• urls
• replies
• normal text

Also allows for setting a colour (background and/or foreground) on one's own messages (black background is recommended), making conversations easier to follow.

Addresses some of the points in #124, recommended to be merged alongside #294