Make the appearance of the widget styleable

[micHar]

Stylable attributes:

• ~~bg color of the bubble~~,
• width/height of the bubble,
• ~~text style of the bubble~~,
• bubble corner round size,
• ~~animation for bubble showing/hiding~~ (via ScrollerViewProvider),
• ~~bubble hide time offset~~,
• ~~handle color~~,
• ~~custom layout for bubble~~,
• ~~custom layout for handle~~,
• handle margin,
• bubble margin.