Widget\AlertMessageBox: fix broken widget

This widget is pretty useless as the error view script can to it's
job. Interestingly nobody missed it, even the render() call was wrong.

I guess we will remove this soon, but for now it's fixed. Looks ugly,
but works.
Modules\Module: modules should register config tabs

This allows modules to register their config tabs and provides a
convenient way to fetch a modules fully instanced config tabs.
config/devtools: playground for developer tools

This is nothing but a prototype right now but could become pretty
useful. We want to allow our users to control most of our developer
tools without deep knowledge about Icinga Web internals.

This first prototype allows allows one to toggle the JS UI debug
window.
ModuleActionController: module object shortcut

This helps modules with the need to know more about themselves. No
more App -> Module Manager -> Module voodoo.
Widget\FilterEditor: handle custom vars

There was an issue with custom vars as they made only partially part
of the column dropdown. Sorted columns and tried to beautify column
names.

fixes #6569
Widget\FilterEditor: correct expression output

Filter expressions should be filled into form fields exactly as we
expect them at parse time.

fixes #6568
monitoring/configuration: provide a config tab
ConfigController: notifications before redirection

That's how they reach the browser in a nice way
monitoring/list/hosts: support addcolumns parameter

At least it doesn't fail right now, this only works for services so far.
View script still needs to be adjusted, will probably be done with #6541.
monitoring/config: make use of Config()

Thee helper function supports lazy loading, requires less code and is
easier to read.
monitoring/show/history: fix html on empty list

This view script used to render broken HTML on empty result
ConfigController: useless use of render
monitoring/ConfigController: use notifications
ConfigController: switch over to new config tabs

Module configuration now takes place in module detail view and does no
longer pollute our config tabs. Modules are now able to provide a bunch
of own config tabs, we don't really care what they use them for.