TinyMCE Button & Visual Shortcode Replacement

Added a TinyMCE Button to add shortcode user-friendly and created a
visual shortcode replacement inside the editor, similar to the
gallery-shortcode, with handle-buttons for changing the current
shortcode, editing the related gridster and deleting the shortcode from
the content.
Added Pagination & Search

Added Pagination to post-lists available as gridster-widgets and added a
search to post_type blocks, to look for posts (and pages and custom
post_types) usable as gridster-widgets
Updated german translation
updated readme