Add high-resolution variant of FreeCAD document icon resources for al…

…l platforms

We did not have document icons defined for OS X and the existing legacy
icons were low resolution.  For this change, we have created a
freecad-doc.svg icon that serves as the basis for generating
platform-specific FreeCAD document icon sets for the supported
platforms.  Extended the OS X bundle definition to include the
high-resolution document icon set and regenerated the existing 64x64
Gui/Icon/freecad-doc.png from the new svg variant.