Add some Label methods requiring pango consts.
Add support for GTK 3.12.

GTK 3.12 is now the default build target.  Use the build tag gtk_3_10
to continue builds targeting the older 3.10 release.
Fix imports for GTK 3.6-3.10 builds.
GTK 3.6-3.10 needs unsafe too.
Use zerod userData instead of nil for closures.

The plan is to eventually save the entire closure context in memory
allocated through cgo to simulate weak references and avoid a circular
dependancy.
Add initial Popover bindings (for GTK >= 3.12).

The bindings depending on unimplemented GMenuModel and GdkRectangle
bindings have been marked as TODO for now.
Add FileChooserButton type and its New func.