Fix centered & right aligned TextEdit (#8082)

A couple improvements to centered and right-aligned text edits:
- Fix text selection in centered and right aligned text edits
(ironically, this broke in #8076)
- Fix cursor movement in centered and right aligned text edits
(horizontal cursor position will be retained on vertical movement)
- Multiline text edit exceeding available width if there are atoms
- Added atoms & alignment options to text edit demo
- Improve how vertical_align and horizontal_align are applied
- Textedit atom is grow now, removing the need for the extra seperate
grow atom
- This allows us to apply the `align` on the text edit atom instead of
the whole AtomLayout
  - Fixes https://github.com/emilk/egui/pull/8022
  - Fixes https://github.com/emilk/egui/issues/7999

crates/epaint/src/text/text_layout_types.rs

@@ -1244,7 +1244,8 @@ impl Galley {
 
             let new_layout_cursor = {
                 // keep same X coord
-                let column = self.rows[new_row].char_at(h_pos);
+                // char_at is Row-relative, so subtract the row's position
+                let column = self.rows[new_row].char_at(h_pos - self.rows[new_row].pos.x);
                 LayoutCursor {
                     row: new_row,
                     column,
@@ -1266,7 +1267,8 @@ impl Galley {
 
             let new_layout_cursor = {
                 // keep same X coord
-                let column = self.rows[new_row].char_at(h_pos);
+                // char_at is Row-relative, so subtract the row's position
+                let column = self.rows[new_row].char_at(h_pos - self.rows[new_row].pos.x);
                 LayoutCursor {
                     row: new_row,
                     column,