Add cache flushing options in devel_init()

This patch adds options (selectable from admin > settings > devel) to flush the menu cache and the filter cahce on each page load by someone with access devel information permissions.

It is for all those moments when you are developing a module, but cannot figure out why function X does not get parameter Y parsed, only to finc out that was due to a cached menu. Another case would be, when you want to develop and devug a filter, without having to have a SSH/phpmyadmin open, to flush the filter cache everytime.

It also replaces some slects (Disable Enable in a select is very bad UI design) with checkboxes.