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/Mod/TechDraw/Gui/AppTechDrawGui.cpp

@@ -27,7 +27,6 @@
 
 #include <Base/Console.h>
 #include <Base/Interpreter.h>
-#include <Base/Tools.h>
 #include <Gui/Application.h>
 #include <Gui/Language/Translator.h>
 #include <Gui/WidgetFactory.h>
@@ -90,7 +89,7 @@ void loadTechDrawResource()
         {"osifont-lgpl3fe.ttf", "osifont-italic.ttf", "Y14.5-2018.ttf", "Y14.5-FreeCAD.ttf"});
 
     for (auto& font : fontsAll) {
-        QString fontFile = Base::Tools::fromStdString(fontDir + font);
+        QString fontFile = QString::fromStdString(fontDir + font);
         int rc = QFontDatabase::addApplicationFont(fontFile);
         if (rc < 0) {
             Base::Console().Warning(