Spreadsheet Fix '=' regression in spreadsheets (#22146)

* Tools: Add ExpressionLineEdit to QtDesigner plugin

* Spreadsheet: Only use validator when when prefix is not '='

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Co-authored-by: wmayer <wmayer@freecad.org>

src/Mod/Spreadsheet/Gui/SpreadsheetView.cpp

@@ -148,6 +148,8 @@ SheetView::SheetView(Gui::Document* pcDocument, App::DocumentObject* docObj, QWi
     // Set document object to create auto completer
     ui->cellContent->setDocumentObject(sheet);
     ui->cellAlias->setDocumentObject(sheet);
+
+    ui->cellContent->setPrefix('=');
 }
 
 SheetView::~SheetView()