Click again on a member of a selected group to pick it on its own

Clicking an item of a group selects the whole group (#1070). Clicking
again on one of its items is meant to select just that item, to edit it
on its own -- what discussion #1070 proposed -- but the second click
selected the whole group again: Qt left only the clicked item selected on
release, and the group completion pulled the others back in.

A press on a member of a group that is selected whole now notes that
member (ItemGroups::memberToPick(), which also finds the member when the
click lands on a symbol's own text). If the click ends without a drag and
Qt has left only that member selected, the selection stays so. A drag
still moves the whole group; Ctrl+click keeps its meaning; a group of one
is not picked from.

In the GUI, on two grouped texts: one click then Delete removes both
(master and this); click, click again, Delete removes only the clicked
text here, both on master; dragging after one click moves both texts by
the same amount on both. tst_itemgroups: 5 new checks; removing the
whole-group or the group-of-one condition fails one each. ctest 24/24.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/itemgroups.cpp

@@ -62,6 +62,47 @@ QUuid ItemGroups::read(const QDomElement &xml)
 	return QUuid(xml.attribute(QString::fromLatin1(xml_attribute)));
 }
 
+/**
+	@return @a item, or its nearest ancestor, that belongs to a group -- a
+	click on a symbol's own text hits the text, but the symbol is the member
+	-- or nullptr if none does.
+*/
+QGraphicsItem *ItemGroups::groupedItem(QGraphicsItem *item)
+{
+	for (; item; item = item->parentItem()) {
+		if (!groupOf(item).isNull()) {
+			return item;
+		}
+	}
+	return nullptr;
+}
+
+/**
+	@return the member a click on @a hit may pick out on its own: the grouped
+	item hit, when every member of its group is selected already. A first
+	click selects the whole group; a second click, on a member of the group
+	it selected, is how the user asks for that member alone (discussion
+	#1070). nullptr when the click is not that.
+*/
+QGraphicsItem *ItemGroups::memberToPick(QGraphicsItem *hit)
+{
+	QGraphicsItem *member = groupedItem(hit);
+	if (!member || !member->isSelected() || !member->scene()) {
+		return nullptr;
+	}
+	const QUuid group = groupOf(member);
+	int members = 0;
+	for (QGraphicsItem *item : member->scene()->items()) {
+		if (groupOf(item) == group) {
+			if (!item->isSelected()) {
+				return nullptr;
+			}
+			++members;
+		}
+	}
+	return members > 1 ? member : nullptr;
+}
+
 /**
 	Make the selection of @a scene whole groups again after it changed.
 	A group with a selected member is selected entirely, except when the