a11y: Fix for -Wmissing-declarations

a11y: Fix for -Wmissing-declarations

I'll add a bunch of fixes for gcc complaining about
-Wmissing-declarations after finding a bunch of cases today where I had
forgotten to make functions static in the CSS code.

A thorn in those patches is G_DEFINE_TYPE() which doesn't allow making
the get_type() function static, so I added definitions for that function
above the G_DEFINE_TYPE().

After those patches, GTK should compile without warnings when this flag
is enabled.