Improve themes support, with both child and parent themes, updates page title and description

[pedro-mendonca]

This PR checks if the theme exists in WP.org, and if it has parent, if so, download both translation projects, child and parent. Also only shows the themes translations update if the locale isn't en_US, the same logic used in the plugins.

• [x] Check if theme(s) exist in WP.org
• [x] Check if user locale isn't en_US
• [x] If the theme is a child theme (has parent), download both parent and child theme translations
• [x] Add a title and description to the theme updates page
• [x] Tested with WP 6.3

Fixes #35 and #42

[mayukojpn]

Thanks a lot! 🎉 And sorry for the late comment! I'll merge it once I check the codes.