Max window size & other size helpers

[arduano]

Was a bit confused why the max_size API isn't exposed on Window, when it's perfectly functional in Resize. Anyway here's the main thing that it affects:

let screen = ctx.available_rect();
let size = screen.size();

egui::Window::new(self.name())
    .resizable(true)
    .resize(|resize| resize.max_size(size)) // Before
    .max_size(size)                         // After
    .show(ctx, |ui| todo!());

I also added some other relevant helpers for consistency.

This PR doesn't change any logic, only forwards along some helper functions that are already public for consistency.

[Lord-Hellgrim]

This looks very reasonable and I don't see any breaking bugs.

[emilk]

Looks good, but please add #[inline] to the functions

[arduano]

@emilk I'm sorry, which functions? Just the ones I added, or all size related ones, or all of them in the window struct? None of the other functions there have #[inline] so I didn't add it either

[emilk]

Oh, they don't? My bad. All trivial builder functions should have #[inline] on them, but I guess we haven't been great at enforcing that. I can take a pass on fixing that after this PR is merged.

EDIT: Fixed in #3557