[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

Thank you for creating a pull request to contribute to FreeCAD-macros! To integrate your macro please make sure the following steps are complete:

• [x] Please check this box if you're not submitting a new macro.
• [ ] Are you submitting a new macro ?
• [x] Have you followed the 'How to submit a macro' section of the README.md ?
• [x] Your macro has a Description in its header.
• [x] Your macro has a CamelCase name.
• [x] Your macro is named appropriately.
• [x] Your macro contains a Metadata section that immediately follows the header description.
• [x] Your macro is Python3/Qt5 compliant and tested on the latest FreeCAD stable and development releases.
• [x] You're including documentation on how your macro works (bonus: screenshots and/or video on the Wiki)
• [x] Commit message is well-written
• [x] Commit message is titled in the following way [MacroName] Short description.
• [ ] Optional, write or update the changelog in the macro, from latest to oldest.

And please remember to update the Wiki with the features added or changed once this PR is merged.
Note: If you don't have wiki access, then please mention your contribution on the 0.19 Changelog Forum Thread.

---

[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