To reduce compile-times and avoid some overhead to binary size, this will modify some of our
generic functions to use non-generic inner functions where possible. The inner functions are
marked carefully with `#[inline(never)]` to prevent being inlined by LLVM at their callsites
While looking for generic functions to optimize, I have also taken the opportunity to annotate
public non-generic getters and setters with `#[inline]` to ensure that LLVM will inline them
across crate boundaries. By default, only generic functions are automatically inlined, and
only when enabling fat LTO are constant functions reliably inlined across crate boundaries.
This is needed for things like `xdg-desktop-portal` to get the app ID
from a pid, in unsandboxed apps.
https://systemd.io/DESKTOP_ENVIRONMENTS documents this, and this sets
things similarly to how Gnome does. Which should be good for now.
Making `spawn_desktop_exec` an `async` function is reasonable given it
is called in `async` functions in
`cosmic-app-list`/`cosmic-launcher`/`cosmic-applibrary`.
