Add drag-to-open for collapsible panels (#8363)

A fully collapsed `show_collapsible` panel now leaves a thin grab handle
at its fixed edge, invisible until hovered. Dragging it out past
`min_size` (or double-clicking it) reopens the panel. Opt out with
`panel.drag_to_open(false)`.

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/emath/src/range.rs

@@ -170,6 +170,14 @@ impl From<&RangeInclusive<f32>> for Rangef {
     }
 }
 
+/// Makes specifying size ranges slightly more convenient (no need for the extra `.0` suffixes)
+impl From<RangeInclusive<i32>> for Rangef {
+    #[inline]
+    fn from(range: RangeInclusive<i32>) -> Self {
+        Self::new(*range.start() as _, *range.end() as _)
+    }
+}
+
 impl From<RangeFrom<f32>> for Rangef {
     #[inline]
     fn from(range: RangeFrom<f32>) -> Self {