This is how the menu looks like after this commit:
┌────┐
│Game│ Help
├────┴─────────────────────────────┐
│ New game F2 │
│ Toggle pause P │
├──────────────────────────────────┤
│ Quit Alt+F4 │
└──────────────────────────────────┘
Rip that bandaid off!
This does the following, in one big, awkward jump:
- Replace all uses of `set_main_widget<Foo>()` with the `try` version.
- Remove `set_main_widget<Foo>()`.
- Rename the `try` version to just be `set_main_widget` because it's now
the only one.
The majority of places that call `set_main_widget<Foo>()` are inside
constructors, so this unfortunately gives us a big batch of new
`release_value_but_fixme_should_propagate_errors()` calls.
