CurrentColorWidget

As a first step towards color palette, each document window needs a current color widget. It may be placed e.g. in the bottom left corner instead of the "scale when resizing window" button which is to be removed (this must be a global setting in PreferencesDialog). Here's what we need to do:

• The widget itself is just two small concentric rectangles; the inner displays fill color, the outer, stroke color. If there is transparency, a checkered background should show through. It must also display gradients and other fill types. (can we use a small arena for this, with the actual SVG template rendered with the current style?)

• The widget reflects the current color of the fill and stroke, which are properties local to SPDesktop. It is stored as RGBA values (displaying patterns and gradients may be added later). The widget responds to the SetColorSignal to change its displayed color.

• Click on the current color widget must bring up the fill&stroke dialog with fill tab. Shift+click must show the stroke tab.

Also each desktop must store the current stroke style (or only width?). Show and make it selectable (drop-down) somewhere near the current color widget.