Syntax for repeated elements and conditional elements

[ogoffart]

Currently

for tile[i] in memory-tiles: MemoryTile {
   // ...      
}

if foo.some-conditoin : SomeElement {
}

It was suggested in https://github.com/slint-ui/slint/discussions/1818#discussioncomment-4182871 to add brace to extand for more elements:

for tile[i] in memory-tiles {
        MemoryTile {
            // ...
        }
        // potentially more elements
    }
}

if foo.some-condition {
    SomeElement {
    }
    SomeOtherElement {
    }
}

It is assumed that this new scope is not in iteself an element, and when used in layout or other components that do something with their direct children the sub-element are considered direct children.

Unresolved questions:

• Should the old syntax be deprecated or be kept as a shortened version when there is only one element? (after all, this would otherwise add a level of indentation for the common case)
• Can property be declared in the for or if scope? If yes, how do we reference them if we want them to be qualified (we don't)

[ogoffart]

Example of code which would benefit of condition with several elements: https://github.com/slint-ui/slint/blob/85df47966e0e1a36c7a0de83b0c0bbd1d582391a/examples/printerdemo/ui/printer_queue.slint#L184

[tronical]

Whether or not to allow multiple elements is another decision in itself that we could take. We could also say that only one child is allowed, in the event that we can't come up with clear semantics yet for multiple.