Fix binding loop in Spinbox

There is currently a loop:

• upRect changes whenever the size of the background changes
• when upRect changes we adjust both margins and minimum size. This adjusts the size of the background
• upRect changes whenever the size of the background changes

Our requirements are:

• we want the size hint from this once to calculate a minimum size and margins
• we need the position constantly updated so the up indicator can be in the right place

Using two properties allows us to separate our two requirements

BUG: 446354