1. Add methods for coordinate translation between cairo/GtkWidget
coordinates and LdDocument coordinates.
2. Draw the grid correctly (also make it significantly faster).
3. Place the canvas into a GtkScrolledWindow and register
the set_scroll_adjustments signal in the GtkWidget base class.
4. Allow scrolling inside of an area of 200 x 200 units (for now).
In the future, it should be possible to go to infinity.
The previous method of containment, that is using GHashTable,
did not list children in any particular order.
Also names of children were duplicated. One copy has been
stored as a hash table index and another was present in the
child itself.
This step is needed, since the current design is quite chaotic
and it would not be possible to finish the application.
There's still a lot to be done.
After this step, it's time to implement the rest of LdLua.
