Create Message attached property to propagate parameters like room, timeline, index and maxContentWidth down to the message content avoiding lots of boilerplate
We need a check here, because stickers (and really, any images without a
tempSource) will try to access an undefined object.
This fixes the error:
"qrc:/qt/qml/org/kde/neochat/timeline/ImageComponent.qml:106: TypeError: Cannot read property 'source' of undefined"
If we do not set the width/height for stickers (which don't have any)
then the height is okay, but the message has the maximum width which
looks odd.
Instead, let's limit all stickers to 256px and it makes them look much
nicer in chat.
