Added a proposal for a Sidebar control.

redo/proposals/sidebar.md

@@ -0,0 +1,32 @@
+# Sidebar Control
+
+```go
+type Sidebar interface {
+	Control
+
+	AppendCategory(text string)
+	DeleteCategory(index int)
+
+	AppendItem(category int, name string)
+	DeleteItem(category int, index int)
+
+	Selection() (category int, index int)		// or Selected()?
+	Select(category int, index int)
+
+	OnSelected(func())
+}
+```
+
+Simple two-level sidebars.
+
+Could have images on each item in the future.
+
+## Mac OS X
+Source List NSTableView (need to see how this will work)
+
+## GTK+
+GTK_STYLE_CLASS_SIDEBAR (available in 3.4); see how GtkPlacesSidebar implements this
+	- other programs that do: Rhythmbox
+
+## Windows
+????