chat: cleaning + some improvments: - code cleaning, removed some dead code - some improvments on the way size is calculated, removed unnecessary sizing methods which were linked to properties - image have now a max size, this avoid gigantic image in the whole screen - in SimpleXHTMLWidget, Label are now splitted when xhtml is set - use a DelayedBoxLayout for messages, as they are really slow to be resized - use of RecycleView has been investigated, but it is not currently usable as dynamic contents are not propertly handled (see https://github.com/kivy/kivy/issues/6580 and https://github.com/kivy/kivy/issues/6582). Furthermore, some tests with RecycleView on Android don't give the expected speed boost, so BoxLayout still seems like the way to go for the moment. To be re-investigated at a later point if necessary.

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