Improvements for working in the SQL Execution area using keyboard

This addresses two issued mentioned in #2215
# It's possible to focus out of the SQL Editor using Ctrl+PgDown (not used
by QScintilla)
# Ctrl+Tab and Ctrl+Shift+Tab works in all the widgets in the area and
provides the same functionality that the one provided by QTabWidget
(switch for- and backwards through the tabs)

src/sqltextedit.h

@@ -18,6 +18,9 @@ public:
 
     static SqlUiLexer* sqlLexer;
 
+private:
+    void transferFocus();
+
 public slots:
     void reloadSettings();
     void toggleBlockComment();