Export module egui::frame (emilk#5087)

Remove the crate visibility of the frame module. Useful at least when
using `Frame::begin` as otherwise the returned type is opaque to library
users and prevents from creating containers that use `Frame` with a
similar interface.

Alternative is to only export `frame::Prepared` as `PreparedFrame` or
something, but I saw that other submodules of containers are already
public.

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* Closes emilk#2106
* [x] I have followed the instructions in the PR template

crates/egui/src/containers/mod.rs

@@ -5,7 +5,7 @@
 pub(crate) mod area;
 pub mod collapsing_header;
 mod combo_box;
-pub(crate) mod frame;
+pub mod frame;
 pub mod panel;
 pub mod popup;
 pub(crate) mod resize;