Added a document outlining gains when moving up to newer versions of APIs. It currently has a complete GTK+ section, and only headers for Windows and Mac OS X.

compatibility.md

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+# Useful things in newer versions
+
+## Windows
+### Windows Vista
+
+### Windows 7
+
+### Windows 8
+
+### Windows 8.1
+
+## GTK+
+TODO what ships with Ubuntu Quantal (12.10)?
+
+### GTK+ 3.6
+ships with: Ubuntu Raring (13.04)
+
+- GtkEntry and GtkTextView have input purposes and input hints for external input methods but do not change input themselves
+	- according to Company, we connect to insert-text for that
+- GtkLevelBar
+- GtkMenuButton
+- **GtkSearchEntry**
+
+### GTK+ 3.8
+ships with: Ubuntu Saucy (13.10)
+
+Not many interesting new things to us here, unless you count widget-internal tickers and single-click instead of double-click to select list items (a la KDE)... and oh yeah, also widget opacity.
+
+### GTK+ 3.10
+ships with: Ubuntu Trusty (14.04 LTS)
+
+- tab character stops in GtkEntry
+- GtkHeaderBar
+	- intended for titlebar overrides; GtkInfoBar is what I keep thinking GtkHeaderBar is
+- **GtkListBox**
+- GtkRevealer for smooth animations of disclosure triangles
+- GtkSearchBar for custom search popups
+- **GtkStack and GtkStackSwitcher**
+- titlebar overrides (seems to be the hot new thing)
+
+### GTK+ 3.12
+not yet in Ubuntu Utopic (14.10)
+
+- GtkActionBar (basically like the bottom-of-the-window toolbars in Mac programs)
+- gtk_get_locale_direction(), for internationalization
+- more control over GtkHeaderBar
+- **GtkPopover**
+	- GtkPopovers on GtkMenuButtons
+- GtkStack signaling
+- **gtk_tree_path_new_from_indicesv()** (for when we add Table if we have trees too)
+
+## Cocoa
+### Mac OS X 10.7+
+
+### Mac OS X 10.8+
+
+### Mac OS X 10.9+