attempt to fix panel context menu not appearing on empty slot

- The white background is a workaround for not being able to set the
exact color of gray for the XY/Z widgets.

Also clear the ListBox when clearing the grid

Support for G2/G3 arcs in preview window

Limited loading of OpenSCAD files

Removed print statement + clear model list when loading file (probably should add a dirty flag as well to avoid accidentally overriding changes)

Print time guesstimation on file load

Added OBJ to the types of files that can be opened.

Added final semicolon after final OBJ in the file filter