Add template helper function for pango markup escaping

[un-def]

I use playerctl metadata with i3bar and forced to use none markup mode because any special char (<, >, &, etc.) in metadata field breaks pango markup.

It would be convenient to have some helper function to encode special chars to corresponding entities (e.g., < → <). This would allow to use pango markup safely, for example:

playerctl --follow metadata --format '<b>pango_escape({{artist}})</b> — pango_escape({{title}})'

I guess this can be done quite easily with g_markup_escape_text().

[acrisci]

Yeah seems like a good feature. I also use playerctl with i3bar so I would use this.

Instead of pango_escape, I would call it markup_escape or maybe xml_escape and the format string would look like this:

'<b>{{markup_escape(artist)}}</b> — {{markup_escape(title)}}'

[acrisci]

It's in latest and has tests but needs documentation.

[un-def]

It's great! I just did a quick check, the new helper works well.