min-max map unit scale support

This patch adds the option of setting the minimum and maximum scales for sizes in map units. Within this range, the sizes will scale according to the current map scale as usual. Outside that range, the screen dimensions of the sizes will remain constant (as when selecting millimeters as unit).

It is implemented by displaying a small configure button next to the output unit combobox. This button is only visible when map units is selected, and clicking on the button displays a dialog window for entering the min/max scales.

Funded by SO!GIS, Kanton Solothurn