Set the main window title to the name of the current page
This makes it clearer which page you're on when looking at the taskbar, tabbing through applications and so on.
Stuff I noticed but won't be changed until later:
- The title will still duplicate on desktop in the Kirigami header bar (that's a greater Kirigami issue though.)
- Some of the page titles are too generic, like a thread is just called "Thread" but could probably be something like "Thread by Carl Schawn"
- This doesn't affect the settings window