Feature: make dialogs extensible

[lehnerchristian]

Obsolete Description:

Dialogs are currently very limited in terms of interaction possibilities. there is only one hard-coded button in the footer and no possibility to inject more buttons in the footer exists.

see also https://github.com/phovea/phovea_ui/blob/develop/src/dialogs.ts#L19-L33

we need to have a dialog which can handle at least multiple buttons (or everything we want to inject, such as multiple form elements, etc.)

Edit:
it is actually possible to add buttons inside the footer by accessing the footer (see https://github.com/phovea/phovea_ui/blob/develop/src/dialogs.ts#L60-L62) and then adding the elements.

however I've added two other features:

• adding custom CSS classes (e.g. for different modal sizes)
• a getter for the header element such that it is also customizable like the body and the footer