Add dialog allowing you to alter the scenario's graphics sheets from within the scenario editor

- You can copy/paste images into the sheet, or import/export to/from png files

Also:
- Picture controls in the dialog engine have a new "scaled" flag; if set, the picture will be scaled into the provided bounds rather than overflowing. Currently, only full sheets honour the setting.

src/scenedit/scen.core.hpp

@@ -1,6 +1,7 @@
 
 class cDialog;
 
+void edit_custom_sheets();
 void edit_custom_pics_types();
 bool edit_ter_type(ter_num_t which_ter);
 bool edit_monst_type(short which_monst);