Introduce basic page controllers (mvc)

There are number of goals for the initial step of MVC code refactoring:

• improve structure/readability/maintainability of PHP code directly related to handling of forms, lists, etc
• introduce page controllers, so it should be absolutely clear what code is used when we execute a particular action (for example, host creation)
• forms should always specify an action by setting variable 'action' using POST or GET (preferably); for example, proxies.php?action=create
• improve validation; for example, introduce support of array validation
• get rid of use of the global variables such as _POST, _GET, _REQUEST, etc

Next step might be refactoring of page_header,php and page_footer.php as well as creation of a single front-controller.