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.
Previously the argument order for Margins was (left, top, right,
bottom). To make it more familiar and closer to how CSS does it, the
argument order is now (top, right, bottom, left).
This patch adds a GUI::TabWidget to the main UI and allows having
multiple images open at the same time.
Some of the changes here are a bit hackish and mechanical and there's
still code around that needs more work to fit better in the new world.
One nice side-effect of this change is that ImageEditor now always
has one Image associated with it, and it never changes.
