File dialogs should default to the loaded map's directory

[zilov]

Currently, each file dialog (Load map, Save state, Load state, Save agp, Load agp) remembers its own last-used directory independently.

Steps to reproduce:

1. Open maps/tolid_1/tolid_1.pretext via Load map
2. Open the Save state dialog — it opens the home directory instead of maps/tolid_1/
3. Save the state to maps/tolid_1/, then close the dialog
4. Open maps/tolid_2/tolid_2.pretext via Load map
5. Open the Save state dialog again — it opens maps/tolid_1/ instead of maps/tolid_2/

It would improve the workflow if all file dialogs defaulted to the directory of the currently loaded map.