Remove alphabetic sort order of baud rate combobox

Remove ability to click on Z +/- 100. Fixes #86

Added 250000 option for baud rate

Added visual indicator for increment values on mouseover

New UI for XYZ controls of Pronterface

Fix that enable/disable does not immediately redraw. Fix that buttons are not disabled on disconnect.

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)