Rewrite admin section

This is a pretty large change, since it rewrites large parts of the admin screens. I believe this is a big step forward in cleaning up and improving the old code, without breaking compatibility.

Please note that the pull request has a lot of commits, but a portion of those are reversed later, since I made commits while developing. I'll make an effort to have a clean git history in the future.

Because there are a lot of changes, I will break them up in parts. These changes are mostly behind the scenes.

General

• Admin pages now extend an interface. This improves the loading of the different pages.

• A lot of old pages are now refactored to be one class in one file.

• All edited/converted files now follow the WordPress naming conventions.

• A lot of HTML was cleaned up, and deprecated tags were removed.

• Existing classes were converted to the new system and to best practices (private variables and functions were possible).

• A lot of JavaScript was improved.

• The installer was refactored to use classes.

• The image uploader was improved.

• This also includes a fix for the widgets that produced an error in the upcoming WordPress 4.3.

• This also include translations updates for Dutch, and an update pot file.

Gallery/image managing

• The gallery/image managing pages are completely rewritten, using PHP5 and the WordPress API.

• The relevant files were moved to their own package.

• For the user this means the managing pages now behave exactly as built-in WordPress.

• For the user there are now also working screen options, with the WordPress Screen Options API.

• The row actions (e.g. to make custom thumbnails) were also rewritten with new JavaScript.

• There are now many advanced options to make thumbnails: rotating, zooming, etc. This fixes issues #21, #27 and #28 (requested by users).

• Various database statements were optimized to be much faster. I will improve this even more in the future.

Database upgrades

• Cleaned up the database upgrade notifications.

Backwards compatibility & performance

• These changes are fully backwards compatible. Changed functions only have additional, optional parameters. All filters and actions will work as before.

• There are some changes to URLs in the admin section, but I didn't find any plugin the linked directly, so it should be fine.

• These changes have no noticeable impact on performance or memory usage, and they make parts of the code a lot more readable.

If a conflict occurs due to the merging of another pull request, I'll update this one. I have added inline PHP documentation as well, so it should be clear what everything does. If you do have questions, feel free to ask.

I have tested the changes and did not find errors, but if you do find them, feel free to tell me, so that I can fix them.