Fix #1045: crash on selecting an element with the online-installer Qt

Since #983, projectDataBase::newQuery() checked a query with
sqlite3_prepare_v2() and sqlite3_stmt_readonly() on the handle of the
QSQLITE driver. Those calls go to the libsqlite3 QElectroTech links. The
QSQLITE plugin of the Qt online installer does not use that library: it
carries its own copy of SQLite, so the handle belongs to another library
and the call crashes. #1021 then put newQuery() on every element
selection, which is where #1045 hits it.

The check now runs the query with PRAGMA query_only set, through the
driver. SQLite refuses a write itself, before touching a row, so the CTE
prefix #983 closed ("WITH x AS (SELECT 1) DELETE FROM element") stays
closed. A refused or failed query comes back empty, because several
callers call exec() again on what newQuery() returns, after query_only
is off.

QElectroTech no longer calls the SQLite C API anywhere.

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

sources/dataBase/projectdatabase.h

@@ -29,7 +29,6 @@ class QETProject;
 class Diagram;
 class Conductor;
 class Terminal;
-struct sqlite3;
 
 /**
 	@brief The projectDataBase class
@@ -156,12 +155,6 @@ class projectDataBase : public QObject
 				  m_cascade_remove_conductor_query,
 				  m_cascade_remove_element_query;
 
-	public:
-		// Deliberately outside the QET_EXPORT_PROJECT_DB guard below:
-		// newQuery() needs the raw connection to ask SQLite whether a
-		// query only reads, and that check runs in every build.
-		static sqlite3 *sqliteHandle(QSqlDatabase *db);
-
 #ifdef QET_EXPORT_PROJECT_DB
 	public:
 		static void exportDb(projectDataBase *db,