Initial icon set

[spacekookie]

The following functions are available via the Header bar

1. Create new objects (files, chapters, ...) (menu)
2. Delete current object
3. Change formatting settings (menu)
4. Change alignment settings (menu)
5. Choose writing mode (menu)
6. Hamburger-style application menu
7. Save As (menu)
8. Save

The Gtk standard icons aren't exactly nice. So…find some that do, make some from scratch? How do we deal with bright/ dark colour schemes?

cc/ @wieueberall