Fixed #4425 Variable is initialized as string but later written to as array, throws fatal error in PHP 7.1

Added "DATE" form type that works like the "DATETIME" type incl. date picker, just without time details.

This reverts commit 9d3f2bbc.

This reverts commit aab7e05b.

- made placeholders visible for apache, too
- added event raise at formdef loading (on_before_formdef and on_after_formdef)
- added possibility to put plugin files into module/lib/plugin.d/ directory