Bug2064: ESC key should abort drags in time ruler on Linux...

... Reimplement the ESC key handling in TrackPanel and time ruler on all
operating systems so that it does not rely on the focused window, but instead
uses the application-wide event filter.

This includes reversion of 9491605cfc

src/CellularPanel.h

@@ -57,10 +57,6 @@ public:
    
    virtual void UpdateStatusMessage( const wxString & )  = 0;
    
-   // Whether this panel keeps focus after a click and drag, or only borrows
-   // it.
-   virtual bool TakesFocus() const = 0;
-   
 public:
    // Structure and functions for generalized visitation of the subdivision
    struct Visitor {
@@ -163,6 +159,8 @@ protected:
 private:
    struct State;
    std::unique_ptr<State> mState;
+
+   struct Filter;
    
    DECLARE_EVENT_TABLE()
 };