Add Panel to replace SidePanel and TopBottomPanel (#5659)

This combines `SidePanel` and `TopBottomPanel` into a single `Panel`.

The old types are still there as type aliases, but are deprecated.

`.min_width(…)` etc are now called `.min_size(…)` etc.

Again, the old names are still there, but deprecated.

(edited by @emilk)

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

Author: sharky98

crates/eframe/src/epi.rs

@@ -137,7 +137,7 @@ impl CreationContext<'_> {
 pub trait App {
     /// Called each time the UI needs repainting, which may be many times per second.
     ///
-    /// Put your widgets into a [`egui::SidePanel`], [`egui::TopBottomPanel`], [`egui::CentralPanel`], [`egui::Window`] or [`egui::Area`].
+    /// Put your widgets into a [`egui::Panel`], [`egui::CentralPanel`], [`egui::Window`] or [`egui::Area`].
     ///
     /// The [`egui::Context`] can be cloned and saved if you like.
     ///

crates/egui/src/containers/mod.rs

@@ -1,4 +1,4 @@
-//! Containers are pieces of the UI which wraps other pieces of UI. Examples: [`Window`], [`ScrollArea`], [`Resize`], [`SidePanel`], etc.
+//! Containers are pieces of the UI which wraps other pieces of UI. Examples: [`Window`], [`ScrollArea`], [`Resize`], [`Panel`], etc.
 //!
 //! For instance, a [`Frame`] adds a frame and background to some contained UI.
 
@@ -27,7 +27,7 @@ pub use {
     frame::Frame,
     modal::{Modal, ModalResponse},
     old_popup::*,
-    panel::{CentralPanel, SidePanel, TopBottomPanel},
+    panel::*,
     popup::*,
     resize::Resize,
     scene::{DragPanButtons, Scene},