[API] Make `Ui::set_cursor()` public

**Describe the solution you'd like**
The `set_cursor()` API is used internally to directly set the effective area of the current Ui. This can be very useful for complex/nonstandard layouts so I believe it should be made public.

It's a low-level api, sure, but egui doesn't generally shy away from those so in my opinion `set_cursor` should be no exception and just be described as such in the docs.

**Describe alternatives you've considered**
From a user perspective the only real alternative, I believe, is to use the various `Area`, `Window`, `Ui::new()`, `Ui::new_child()` and so on. This can however be sub-optimal due to compounding complexity with deeply-nested `Ui`s, unwanted features in the more high-level alternatives and potentially worse performance. There's also a few cases in which they're simply not sufficient without annoying workarounds.

From an API design perspective, perhaps we could also consider a few user-friendly wrappers like `fn add_margin(f32)`, a cross-axis counterpart to `add_space` that shifts the cursor horizontally for vertical `Ui`'s and vice versa. Not really sure, curious to know what others think.

Start by locating the existing Ui::set_cursor() entry point and read how it is currently documented and exposed internally. Make the API publicly accessible and describe its low-level behavior in the docs, then verify that the public API builds and existing tests pass.

Written by the indexing model from the issue text.