LibGUI: Make descendants of AbstractView define their own select_all() (#1201)

AbstractView does not know which column it's displaying which makes it
impossible to implement the select_all functionality up there. Now
descendants override the pure virtual select_all method and implement
it themselves.

Libraries/LibGUI/ListView.h

@@ -52,6 +52,7 @@ public:
     int model_column() const { return m_model_column; }
     void set_model_column(int column) { m_model_column = column; }
 
+    virtual void select_all() override;
 private:
     ListView();