Add project name to MainWindow

[stefanhahmann]

The current MainWindow is too small to show the project name in its title (at least so in Windows OS)

cf.:

This makes it difficult in settings where multiple Mastodon projects are open simultaneously to identify to which open project the main window belongs.

There are 3 alternatives how to solve this:

1. Make the MainWindow wider
2. Make the MainWindow resizable
3. Show the project name not only in the title, but also inside the MainWindow

This PR implements option 3 (not claiming that it is the best out of these options).

After PR: