chat: design improvments: - all message are now on the same side, no more left/right which can be difficult to read on big screen - use of GridLayout for MessageWidget and MessagesWidget, as the minimum_height attribute simplify size handling. This attribute will be present in BoxLayout too in Kivy 1.9.2, so we may switch back in the future - info messages are displayed in yellow with a black background - nick and date are now displayed in the same label - avatar, nick and date are on the same line just above the message bubble - padding/spacing adjustments