Reset device selection on agent swap or unload (#124)

set_live_printer_agent centralizes
the swap: deselect the machine,
clear stale sidebar state and the
previous agent's Other Devices, then
install the new agent (or null when
its provider vanished). Plugin
load/unload callbacks refresh the
dropdown and re-run agent selection.
load_last_machine no longer falls
back to the first available machine.

src/slic3r/GUI/Tab.hpp

@@ -675,6 +675,7 @@ public:
 	wxSizer*	create_bed_shape_widget(wxWindow* parent);
 	void		cache_extruder_cnt(const DynamicPrintConfig* config = nullptr);
 	bool		apply_extruder_cnt_from_cache();
+	void		refresh_printer_agent_dropdown() const;
 };
 
 class TabSLAMaterial : public Tab