Better support for translated pages, removed hidden pages from the sitemap

[tsnorri]

• The plugin generated URLs for translated pages that in some cases led to Page not found errors. This happened in Grav 1.4.5 and Grav 1.6.0-rc3. I wasn’t able to identify the cause so I decided to translate all path components instead.
• Modular pages are intended to be included as part of the parent page, so they should affect its modification date.
  • By default, page->collection() is used to get the subpages, after which modular() is used to retrieve the modular pages, but the modular collections’ names may also be specified in the page’s frontmatter. I didn’t yet test this, though.
• In addition, since the sitemap format only allows for one timestamp per url element (including translations, to my understanding at least), the translations’ modification timestamps should affect the modification date.
• Hidden pages appeared in the sitemap, for which I didn’t see any reason, so I added a check.
• Merged Hydraner’s and CSixtyFour’s modifications.

[rhukster]

I merged the other PR related to this one shows in Draft