Allow replacing existing objects with template instances

[bjorn]

There should be a way to turn an already placed object into a template instance. Essentially the reverse of the "Detach" action that turns a template instance into a regular object.

Of course, the question is which template to use. I think there are two options:

• Add "Replace with Template" context menu action that uses the template selected in the Templates view.
• Add a menu similar to the one shown at issue Custom properties and templates #818, which allows selecting the template in the menu.

I'm somewhat leaning towards the first option.

[bjorn]

@thabetx Is this something you may still get around to?

[thabetx]

This looks like a good idea. I prefer the first option, mostly because it will have the same workflow as replacing the tile of an object. I will look into it.

[bjorn]

Implemented in 6404b3d.