Clamp text cursor positions in the same places where we used to (#7081)

Closes #7077.

This fixes the problem shown in #7077 where clearing a `TextEdit`
wouldn't reset its cursor position. I've fixed that by adding back the
`TextCursorState::range` method, which clamps the selection range to
that of the passed `Galley`, and calling it in the same places where it
was called before #5785.

(/cc @juancampa)

* [x] I have followed the instructions in the PR template

crates/epaint/src/text/text_layout_types.rs

@@ -1104,6 +1104,10 @@ impl Galley {
         }
     }
 
+    pub fn clamp_cursor(&self, cursor: &CCursor) -> CCursor {
+        self.cursor_from_layout(self.layout_from_cursor(*cursor))
+    }
+
     pub fn cursor_up_one_row(
         &self,
         cursor: &CCursor,