Both applications eventually reach LibGPU's loading of a dynamic library
(e.g. LibSoftGPU) which requires this pledge.
The pledge was always required, but went unchecked until 01318d8f9b.
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.
