Add Drop to Bed action for objects, parts, and modifiers

[BenJule]

Summary

Add a Drop to bed action to the object manipulation panel for complete object instances, individual parts, and modifiers.

Bambu Studio already contained the minimum-Z calculation and visibility state for this feature, but the action was never exposed in the ImGui interface and full-instance selections always used a zero minimum height.

Changes

• Add a visible Drop to bed button below the position controls
• Support complete object instances
• Support individual parts and modifiers inside assemblies
• Calculate the correct minimum Z for full-instance selections
• Move selections along the global Z axis onto the build plate
• Preserve undo and redo through the existing gizmo snapshot mechanism
• Hide the button when the selected item is already on the build plate
• Reset the visibility state when the current selection is unsupported

Testing

• git diff --check
• Successfully compiled the affected translation unit:

ninja -C build src/slic3r/CMakeFiles/libslic3r_gui.dir/GUI/Gizmos/GizmoObjectManipulation.cpp.o -j6

• Re-running the targeted build reports ninja: no work to do, confirming the object file is current
• No new compiler warnings or errors were introduced

Fixes #5536
Fixes #9076

[Haidiye00]

Hi, please record the GIF effect after the changes, thanks

[BenJule]

Here's a quick demo of the Drop to Bed action: