Use gridUnit for default panel thickness
Review Request #126363 - Created Dec. 15, 2015 and submitted
This makes the initial panel thickness scale.
I've removed the calculation taking screen res into account because res != physical size, and this is not actually working on my three systems (screenGeometry.height() is always 0), nor have I seen proportionally larger panels on screenshots. If it's not working it means nobody is used to getting a larger panel, nor have there been complaints, so we can drop this complexity. It also means we don't have to work in half-gridUnits to get close to the original value (if you assume the original delta was chosen on a 96 dpi system).
Default panel thickness now comes out as 28px instead of 27px on a 96 dpi test system.