Loader: Implement existing product group picker in Group Products dialog

[BigRoy]

Changelog Description

Implement an "existing product group" picker in Group Products dialog,

Additional info

Fixes #648

Testing notes:

1. Press CTRL+G in loader to get the Group Products dialog
2. If the current folder has any existing product groups - clicking button right hand side in pop-up should list them for you to easily "put it into that group"
3. if the current folder has no existing product groups - the button right hand side should be disabled.

[iLLiCiTiT]

Pre-fill group field value based on selection.

Suggested change

	self._name_line_edit.set_options(list(sorted(product_groups)))
	product_group = ""
	if product_groups:
	product_group = next(iter(product_groups))
	self._name_line_edit.setText(product_group)
	self._name_line_edit.set_options(list(sorted(product_groups)))

[BigRoy]

The listed product groups are ALL groups under the active folders - not just the groups of the products in your selection. Just so that you can easily move it into another group outside of your selection.

So technically, this would first have to get all the product entities, get their product groups - then from those pick the first entry (or most used entry)...

[iLLiCiTiT]

Well, right now it's prefilled with previous value of the widget. So either we use first, or empty string, or most common, but current behavior is not correct.

Suggested change

	self._name_line_edit.set_options(list(sorted(product_groups)))
	self._name_line_edit.setText("")
	self._name_line_edit.set_options(list(sorted(product_groups)))

[iLLiCiTiT]

Reminder...

[robin-ynput]

Confirmed it works locally.