Show a cancel button in modals

[nilmerg]

This must not depend on the content. e.g. a form in the modal must not be required nor must not the form itself be responsible for the button.

Maybe, we should introduce a control area in modals. In this area, the cancel button is default and always aligned to the left. Any other buttons are aligned to the right. This also has the benefit that buttons are always visible.

The base form class (or the IcingaFormDecorator in the case of ipl\Web) could support this natively. There are already various cases for forms, that provide multipart content. It makes sense to add native support for this. Then forms, that are loaded inside a modal, could target the new control area when rendering buttons.