Automatically dismiss unread notifications when the window regains
focus.

fixes #111
fixes #68

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.

Fixes #64

closes #41

closes #11

Basically reverts the last font related commits since pointSize isn't
as reliable as pixelSize.

Also some layout values (margins, spacings) have been moved out to Config.h.

- Fixes small font issue on MacOs

- Rooms without any history will be shown.
- Room's state will be kept in sync and any updates will be visible.

In order to be compatible with the Matrix terminology

Each HistoryView maintains a list of pending events. Each pending
message is validated from the homeserver with either the returned
EventId or the body of the message.

Currently there is no support to remove invalid messages.

Also some small refactoring:
    - ChatPage doesn't know about the message being sent. The message
      delivery is solely handled by HistoryViewManager.
    - Nick coloring function moved to HistoryViewManager.

Tone down the main text color