Add category feature to the core.

Updated robots.txt + new htaccess files.

I made the following changes for the robots.txt and .htaccess files.

# robots.txt updated
- Allow theme assets (CSS, JS, Images, Fonts) for SEO
- Allow folder with uploaded images (/content/images)

# .httaccess files added
- /content - Deny all
- /content/images - Allow only jpg|png|gif
- /cache - Deny all
- /system - Deny all
- /system/admin/editor - Allow all
- /system/resources - Allow all
- /themes - Deny all
- /themes/blog/css - Allow all
- /themes/blog/fonts - Allow all
- /themes/blog/images - Allow all
- /themes/blog/js - Allow all
- /themes/clean/css - Allow all
- /themes/default/css  - Allow all
- /themes\default\img - Allow all
- /themes\logs\css - Allow all

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.

This reverts commit 9fa8fab864.

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.

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

Fix the date and time on the edit content page so it will displayed as
year, month, day, hour, minute, and second.

Add bootstrap menu classes.