support XCL 2.2 editor changer function

- Edited template "main_content_form.html".
- Added new constants for each filters.
define('_MD_PICO_FILTERS_[PluginName]EDITOR', '');           // Editor
type
define('_MD_PICO_FILTERS_[PluginName]CSSCLASS', '');         //
Additional CSS class name for TEXTAREA
define('_MD_PICO_FILTERS_[PluginName]USEHTMLATNEW', false);  // Switch
to HTML editor on makeing new entry
define('_MD_PICO_FILTERS_[PluginName]DISABLEONHTML', false); // Disable
this filter on using HTML editor
-- These constants are not necessarily required.
- Added a new filter "xoopsts" that XOOPS default text filter.