Horizontal scrolling (#663)

* First pass (missing rendering the bar)

* Render horizontal bars, and change Window scroll API

* emath: add impl Index + IndexMut for Align2

* Scrolling: fix subtle sizing bugs

* Add horizontal scrolling to color test

* try to wrap content before showing scrollbars, + add auto-shrink option

* Add hscroll to the misc demo window

* Fix for putting wrapping labels in an infinitely wide layout

* Add a egui_asserts to protect against nans in the layout engine

* Add line about horizontal scrolling to changelog

* Add example to docs of ScrollArea

* code cleanup

egui_demo_lib/src/apps/demo/misc_demo_window.rs

@@ -34,13 +34,16 @@ impl Demo for MiscDemoWindow {
     fn show(&mut self, ctx: &CtxRef, open: &mut bool) {
         Window::new(self.name())
             .open(open)
-            .scroll(true)
+            .vscroll(true)
+            .hscroll(true)
             .show(ctx, |ui| self.ui(ui));
     }
 }
 
 impl View for MiscDemoWindow {
     fn ui(&mut self, ui: &mut Ui) {
+        ui.set_min_width(250.0);
+
         CollapsingHeader::new("Widgets")
             .default_open(true)
             .show(ui, |ui| {