Link a symbol built to its partners from the project database's rows

A symbol built after opening (lazy folios) links to the partners its
link rows name, and to the symbols whose rows name it; its file's list
only adds what the rows miss. The rows keep a partner on a folio not
built until it is built, in its saved place, so a symbol no longer keeps
a list of its own: the list read from the file is dropped once opening
has filled the database. pendingLinks(), waitingLinks(),
keepsSavedLinks(), linksToSave() and QETProject::waitingLinkTargets()
go; the callers that asked a symbol for its waiting partners ask the
database (waitingPartners()).

When its links change, a symbol's rows are written from its built
partners and the waiting partners its rows held, in their order
(rowsToWrite()), so a full rewrite of the table keeps them too.

A symbol built alone as a partner on a folio not built
(Diagram::buildSymbols()) counts as built: its rows are kept, it is not
waiting (waitingSymbols(), waitingPartners()), so saving does not build
its folio, and removing it does not remove the rows naming it. A row
naming a placed symbol that has not linked yet (its folio being built)
is kept too. At opening the database holds no link until its fill
(forgetLinks()); the symbols read their files' lists until then, and the
fill takes their order: a symbol's own rows, its list, then the rows
naming it. The symbols waiting on folios not built are cached and
updated as folios are built.

QET_CHECK_LINK_ROWS now checks the rows against the links: each built
partner in its order, no other row but to a folio not built.

tst_resaveunchanged::undoKeepsLinksToUnbuiltFolios: m_000 opened with
lazy folios, a contact, its folio or its coil's folio (not built)
removed, then undone: every link saved as before.

QETZ-DB-FIRST stage 2a-3.

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

sources/elementprovider.cpp

@@ -22,6 +22,7 @@
 #include "qetgraphicsitem/element.h"
 #include "qetgraphicsitem/terminalelement.h"
 #include "qetproject.h"
+#include "dataBase/projectdatabase.h"
 
 #include <QAbstractItemModel>
 
@@ -111,7 +112,8 @@ QVector <QPointer<Element>> ElementProvider::freeElement(ElementData::Types filt
 		{
 			if (filter & elmt_->elementData().m_type &&
 				elmt_->isFree() &&
-				!elmt_->pendingLinks().intersects(waiting))
+				(waiting.isEmpty()
+				 || m_project->dataBase()->waitingPartners(elmt_->uuid(), waiting).isEmpty()))
 			{
 				free_elmt << elmt_;
 			}