Editor: Refactor Open Link in Next Split.

Cleanup code and added shortcuts and menu entries for
- open header/source in next split
- follow symbol under cursor in next split
- open declaration/definition in next split

Change-Id: I2c4347749d26669d88b7c2968f30f60710f442b1
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/cpptoolsplugin.h

@@ -74,6 +74,7 @@ public:
 
 private slots:
     void switchHeaderSource();
+    void switchHeaderSourceInNextSplit();
 
 #ifdef WITH_TESTS
     void test_codegen_public_in_empty_class();