Move chart list from GdMainView to GtkIconView

This seems to be a more reliable interface, and maybe more suitable
for future needs (like adding a GtkSpinner as the chart image while
rendering preview).

Right now it looks almost the same as before:

* there are still now preview images
* there is, however, a placeholder for them. It is currently 256x256,
  which may be too big for the final version.
* selection mode checkboxes are now visible (a huge step forward!)