404 pages rendered within layout by @greenphp

Before this pull request, the 404 and 404-search templates where handled
as full HTML pages without the page context (layout).

With this pull request both pages are rendered inside the selected
themes layout. Additionally the 404 template now contains the "search
form" and the homepage link.

All Themes are already updated with the above changes.

Set the Sitemap priorities within the config.ini (SEO) by @greenphp

Feature to change Sitemap priorities direct with the "config.ini" for
SEO. Additionally this commit adds the "lastmod" tag for posts.

- Set the Sitemap priorities for subtypes (Base, Post, Static, Tag, ...)
within the config.ini
- Values beween 1.0 and 0.0 or "false" to disable a subtype.
- Default priority is 0.5 as fallback. (see functions.php) The default
for "base" (Homepage) is 1.0.
- Disabled subtypes aren't available in the sitemap.xml (index).
- The sitemap for posts now contains the "lastmod" set the the post's
date.

Make the tag URL always lowercase and accept case insensitive URL e.g
/tag/htmly or /tag/HTMLy.

Cache the widget and refresh it when any content edited or new content
added.

Use htmlspecialchars() for title tag and description tag generation.

Adding popular posts widget.

Add new theme blog