Comment/uncomment code command for the SQL editor

A new command is added for commenting or uncommenting SQL code (depending
of the state of the first line of the block).

It is bound to "Ctrl+/" as used by many IDEs ("Ctrl+-" is more standard as
Zoom out). Unusual Scintilla binding for "Ctrl+/" is removed for avoiding
interference.

If there is no selection, the current line is un/commented.

See issue #1614

src/sqltextedit.h

@@ -23,6 +23,9 @@ public:
 public slots:
     void reloadSettings();
 
+private slots:
+    void toggleBlockComment();
+
 };
 
 #endif