Fix inspector window sometimes not hiding

When the f shortcut is quickly pressed, the inspector window will
sometimes open and remain open even though it should automatically
close. This commit ensures the window closes by capturing the f key
release event from the main window in addition to the inspector window.

rtgui/filepanel.h

@@ -81,6 +81,7 @@ public:
     bool imageLoaded( Thumbnail* thm, ProgressConnector<rtengine::InitialImage*> * );
 
     bool handleShortcutKey (GdkEventKey* event);
+    bool handleShortcutKeyRelease(GdkEventKey *event);
     void updateTPVScrollbar (bool hide);
 
 private: