Simplify the text cursor API (#5785)

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* Closes N/A, but this is part of
https://github.com/emilk/egui/issues/3378
* [x] I have followed the instructions in the PR template

Other text layout libraries in Rust--namely, Parley and Cosmic
Text--have one canonical text cursor type (Parley's is a byte index,
Cosmic Text's also stores the line index). To prepare for migrating egui
to one of those libraries, it should also have only one text cursor
type. I also think simplifying the API is a good idea in and of
itself--having three different cursor types that you have to convert
between (and a `Cursor` struct which contains all three at once) is
confusing.

After a bit of experimentation, I found that the best cursor type to
coalesce around is `CCursor`. In the few places where we need a
paragraph index or row/column position, we can calculate them as
necessary.

I've removed `CursorRange` and `PCursorRange` (the latter appears to
have never been used), merging the functionality with `CCursorRange`. To
preserve the cursor position when navigating row-by-row, `CCursorRange`
now stores the previous horizontal position of the cursor.

I've also removed `PCursor`, and renamed `RowCursor` to `LayoutCursor`
(since it includes not only the row but the column). I have not renamed
either `CCursorRange` or `CCursor` as those names are used in a lot of
places, and I don't want to clutter this PR with a bunch of renames.
I'll leave it for a later PR.

Finally, I've removed the deprecated methods from `TextEditState`--it
made the refactoring easier, and it should be pretty easy to migrate to
the equivalent `TextCursorState` methods.

I'm not sure how many breaking changes people will actually encounter. A
lot of these APIs were technically public, but I don't think many were
useful. The `TextBuffer` trait now takes `&CCursorRange` instead of
`&CursorRange` in a couple of methods, and I renamed
`CCursorRange::sorted` to `CCursorRange::sorted_cursors` to match
`CursorRange`.

I did encounter a couple of apparent minor bugs when testing out text
cursor behavior, but I checked them against the current version of egui
and they're all pre-existing.

crates/egui_demo_lib/src/easy_mark/easy_mark_editor.rs

@@ -146,7 +146,7 @@ fn shortcuts(ui: &Ui, code: &mut dyn TextBuffer, ccursor_range: &mut CCursorRang
     if ui.input_mut(|i| i.consume_shortcut(&SHORTCUT_INDENT)) {
         // This is a placeholder till we can indent the active line
         any_change = true;
-        let [primary, _secondary] = ccursor_range.sorted();
+        let [primary, _secondary] = ccursor_range.sorted_cursors();
 
         let advance = code.insert_text("  ", primary.index);
         ccursor_range.primary.index += advance;
@@ -177,7 +177,7 @@ fn toggle_surrounding(
     ccursor_range: &mut CCursorRange,
     surrounding: &str,
 ) {
-    let [primary, secondary] = ccursor_range.sorted();
+    let [primary, secondary] = ccursor_range.sorted_cursors();
 
     let surrounding_ccount = surrounding.chars().count();