This allows for typing [8] instead of [8, 8, 8, 8] to specify the same
margin on all edges, for example. The constructors follow CSS' style of
specifying margins. The added constructors are:
- Margins(int all): Sets the same margin on all edges.
- Margins(int vertical, int horizontal): Sets the first argument to top
and bottom margins, and the second argument to left and right margins.
- Margins(int top, int vertical, int bottom): Sets the first argument to
the top margin, the second argument to the left and right margins,
and the third argument to the bottom margin.
The FontSettings widget now uses background_role instead of
background_color to ensure that it displays properly independently of
the active system theme.
