fix: distinguish grab and grabbing cursors (#1154)

On macOS, there is a difference between a "grab" cursor and a "grabbing"
cursor, where "grab" is an open-hand cursor used during a hover, and
"grabbing" is a closed-hand cursor used on a click. These, and other
native MacOS cursors, can be seen at the [NSCursor documentation](https://developer.apple.com/documentation/appkit/nscursor?language=objc).

See https://github.com/hecrj/iced/issues/9 for the motivation for this
PR.

src/window.rs

@@ -760,7 +760,9 @@ pub enum CursorIcon {
     Alias,
     Copy,
     NoDrop,
+    /// Indicates something can be grabbed.
     Grab,
+    /// Indicates something is grabbed.
     Grabbing,
     AllScroll,
     ZoomIn,