Be more strict when matching C symbols

[lazka]

In case there is no explicit reference we should only convert matches if they are namespaced to prevent false positives.

e.g.: Only match if it contains "_" or an uppercase character that isn't the first one.

https://lazka.github.io/pgi-docs/#Gtk-3.0/classes/CellRenderer.html#Gtk.CellRenderer.props.visible