Introduce RiaZScaleTools as a single source for the Z-scale factor list, merging the predefined factors with custom values entered during the session. The toolbar combo box, the property editor combo box and the Ctrl+Shift+Up/Down shortcuts all use this shared sorted list.
The toolbar combo box previously appended custom values at the end, producing an unsorted, ever-growing list, and the shortcuts stepped through the predefined factors only, skipping custom values visible in the list.
