ToolPalette example: Skip non-existent and large icons

* examples/book/toolpalette/canvas.h: CanvasItem ctor: get label, if
no icon name.
* examples/book/toolpalette/canvas.cc: Catch Gtk::IconThemeError.
* examples/book/toolpalette/examplewindow.cc: Make the ToolPalette look
better by skipping large icons and icons that can't be loaded from
the current icon theme.
* docs/tutorial/C/figures/toolpalette.png: Updated.
Drag and Drop chapter: Correct the description of the DnD signals

* docs/tutorial/C/gtkmm-tutorial-in.xml: The drag_motion and drag_drop
signals are emitted by the destination widget (were listed under the
source widget).