QML needs better layout support

Checkbox with text on right side:
1) Checkbox-button is of fixed size
2) Implicit size is implicit size of Checkbox-button + implicit size of text + margins
3) Text use as much space as it can, truncating the text if there's not enough space
4) Between the checkbox and the text there's a space if the text is not empty

Button with icon and text:
1) Button may have text and/or icon.
2) Icon is left of text
3) Implicit size of the button is implicit size of icon + implicit size of text + margins
4) If button is not wide enough, the text is truncated
6) If the button is not high enough, the icon is scaled down with preserved aspect ratio
7) If the button is wider or higher than its implicit size, the icon and text is centered. The icon is never scaled up.