setStyleSheet was overriding default tooltip theme from the system,
causing themes like Breeze to render white text on a grey bg for
tooltips. Tooltips now render default for the system theme everywhere.

set a theme matching color on the dark theme comboboxes.

fixes #118

Respect system styling
Increase size of emojis (to remove empty space)
Add hover effect (partially adresses #41)
Less hardcoding of sizes
Use emoji font (color)

When user is mentioned (via matrix 'highlight_count'), inactive
rooms will use a different color for the notification circle than
when only general unread messages exist.

work regardless of the theme choices the user makes.  The code
now incorporates the contrast between the background color and the
color generated by the user_name when picking colors.  It currently
has two 'big' issues:
1. Colors are not cached.  I am planning on adding a QHash for this
a little later.  This should improve performance by not calculating
the color for the same users over and over and over again.
2. Theme changes do not trigger the colors to get refreshed.
Currently, you will have to switch to a different room and back
to get the colors to refresh.

Update dark and system themes to provide
colors for the hover states for title, subtitle, and timestamp.

fixes #204
fixes #255

fixes #248

fixes #40

fixes #245

fixes #128

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.