Base: Drop QString-std::string conversion functions from Tools

Convenience helpers function Tools::toStdString and Tools::fromStdString
were implemented for Qt4 or older to perform utf8 aware conversion as
QString::toStdString/QString::fromStdString were using toAscii/fromAscii
internally (see https://dreamswork.github.io/qt4/classQString.html).

Since Qt5 QString uses toUtf8/fromUTf8, which makes the helper functions
obsolete (see https://doc.qt.io/qt-5/qstring.html#fromStdString).

src/Gui/SpinBox.cpp

@@ -35,7 +35,6 @@
 
 #include <App/ExpressionParser.h>
 #include <App/PropertyUnits.h>
-#include <Base/Tools.h>
 
 #include "SpinBox.h"
 #include "Command.h"
@@ -108,7 +107,7 @@ void ExpressionSpinBox::showValidExpression(ExpressionSpinBox::Number number)
         p.setColor(QPalette::Text, Qt::lightGray);
         lineedit->setPalette(p);
     }
-    iconLabel->setExpressionText(Base::Tools::fromStdString(getExpression()->toString()));
+    iconLabel->setExpressionText(QString::fromStdString(getExpression()->toString()));
 }
 
 void ExpressionSpinBox::clearExpression()