new frame tool

[yzemaze]

I needed a way to easier and faster highlight things. The new tool FRAME allows drawing predefined rectangles around the clicked location – with just one click. Rectangles don’t have perfect (sharp) outer corners on purpose as I was set on using cairo_arc to facilitate rounded corners and drawing circles as well (radius = xlength/2).
With the addition of fillcolor there should be many useful options, including drawing dots (circles with color = fillcolor or size = xlength = ylength).
Due to width already being in use in paint_context_new I opted for xlength (and ylength instead of height). I couldn’t find more descriptive terms.
I decided not to change the default radius of 10 to not mess with orthogonal.