Author color is now cached so that it will not be re-calculated
each time a new message is posted.  This cache gets cleared when
the theme is changed.

Additionally, the author color is now automatically refreshed
when the theme is changed, fixing the issue where you had to
change rooms before the colors would switch.

Theme and Font Family settings will now correctly display the stored
settings when nheko is re-launched.  Previously, these combo boxes
would default to the first thing in the combo box, even if that
wasn't what the user selected.

User can now select a font from the installed fonts on their system

This font currently will only be applied when nheko is restarted
(similar to how font size and scaling currently work).  This will
be addressed in a future commit.  Additionally, the dropdown
does not correctly select the previously-chosen user font, and
instead defaults to the first font available on the system
(alphabetically).  This is similar to the issue with the 'Theme'
combo defaulting to 'Light' even when another theme is selected.

fixes #161

fixes #388

fixes #357
fixes #335
fixes #230

fixes #239

fixes #261

fixes #131

fixes #2

The color scheme of nheko obeys the default color theme of Qt
(i.e. the system theme). It uses a Qt stylesheet to accomplish this,
which means replacing the color theme with a custom theme would only
be a matter of writing a new style sheet and loading it into the app.