Implement and move some colour-aware helper methods into the Theme class

Implement and move some colour-aware helper methods into the Theme class

This introduces a new method to change the colours in the links in QLabel's.
Utilizes a custom crafted RegEx function to replace already-coloured links.

Moved code is based on stuff from the SettingsDialog class.

Signed-off-by: Michael Schuster <michael@schuster.ms>