Improve gcode preview navigation (#5326)

* preview: home/end to move to start/end step of the layer

Add HOME and END key action to preview tab which moves horizontal slider
position to a first and last position respectively

* preview: cross-layer gcode inspection

Automatically advance gcode preview to the next layer when pressing
right at the last step of the layer. Same for the first step of the
layer: pressing 'left' will switch preview to the previous layer

src/slic3r/GUI/KBShortcutsDialog.cpp

@@ -313,8 +313,8 @@ void KBShortcutsDialog::fill_shortcuts()
 		    {L("Ctrl+Any arrow"), L("Move slider 5x faster")},
 		    {L("Ctrl+Mouse wheel"), L("Move slider 5x faster")},
        #endif
-
-        
+        { L("Home"),        L("Horizontal slider - Move to start position")},
+        { L("End"),         L("Horizontal slider - Move to last position")},
     };
     m_full_shortcuts.push_back({ { _L("Preview"), "" }, preview_shortcuts });
 }