[Gtk] Optimize RichTextView Markup support
[Gtk] Optimize RichTextView Markup support

Convert FormattedText.Attributes to Gtk.TextTag
values directly without passing them through Pango
attributes.

akrisiun/xwt,antmicro/xwt,hamekoz/xwt,residuum/xwt,lytico/xwt,mono/xwt,cra0zy/xwt,hwthomas/xwt,TheBrainTech/xwt