[Export2Slicer] Update

[dlozinski]

• Take ActiveObject to export when no selection
• Update default Slicer command with ''--single-instance" Change macro title to 3D Print / Slice
• Set default export format to AMF

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[dlozinski]

Aside code cleaning this PR changes the behavior of a macro that picks whole top level object if nothing is selected.
Second update is to keep preserve the selection during the export