theme-parser: Use peek_required_version() for validation

When validating button functions and frame styles, the required
format version of the features used in the theme was compared to
the major version number of the supported format, limiting additions
to major theme format bumps.
Use peek_required_version() instead, so the minor version number
of the supported theme format is taken into account.

https://bugzilla.gnome.org/show_bug.cgi?id=635683