Support arbitrary action buttons for list item2

[abey79]

Currently, PropertyContent supports only one action button, but it would be nice to have support for more than 1. After discussing with @martenbjork, we should use the following strategy:

• each PropertyContent can have an arbitrary number of action buttons
• the right gutter is sized according to the maximum action button used in a given list item scope
• if that gutter size is too big w.r.t the available content, it can be reduced to be just one button wide
• if that's the case, then item with more than 1 action button will display a … button and show available action in some on-hover/on-tap mini toolbar

[abey79]

Part/related to this issue:

• LabelContent::with_buttons() should be reconciled with the ItemButton abstraction, probably in a similar way as the multi-button discussion above.
• Then, SectionCollapsingHeader::help() should also be implemented using that abstraction, by making a new ItemHelpButton implementation of ItemButton.

[abey79]

• Related List item: LabelContent should be able to have the exact same action button and behaviour as PropertyContent #6203