Fix deprecated QRegExp

Use QRegularExpression instead.

https://doc.qt.io/qt-5/qregularexpression.html#notes-for-qregexp-users

This function was introduced in Qt 5

sources/diagramposition.cpp

@@ -29,7 +29,7 @@
 DiagramPosition::DiagramPosition(const QString &letter, unsigned int number) {
 	// purifie les lettres
 	letter_ = letter.toUpper();
-	letter_.remove(QRegExp("[^A-Z]"));
+	letter_.remove(QRegularExpression("[^A-Z]"));
 	number_ = number;
 }
 

sources/diagramposition.h

@@ -19,7 +19,6 @@
 #define DIAGRAM_POSITION_H
 #include <QPointF>
 #include <QString>
-#include <QRegExp>
 /**
 	This class stores the position of an electrical element on its parent diagram.
 	While exact coordinates can be stored for convenience, the concept of diagram