fix: convert xmap path names for windows

* make sure, unsaved edits only count in editmode (otherwise "watercolour"
  in map configs would already be "edits")

* nicer filenames for backup-xmaps

* catch failed xmap file write access

* unsavededits is now flag (!= 0) and counter - to enable scripts to
  detect "new edits".

new commands:

xmap_delete_backup    deletes the backup xmap

xmap_keep_backup    moves the backup xmap to a regular named spot

xmap_rename    changes the nickname of an xmap

source/src/worldlight.cpp

@@ -174,7 +174,7 @@ void fullbrightlight(int level)
     lastcalclight = totalmillis;
 }
 
-VARF(ambient, 0, 0, 0xFFFFFF, if(!noteditmode("ambient")) { hdr.ambient = ambient; calclight(); unsavededits = 1;});
+VARF(ambient, 0, 0, 0xFFFFFF, if(!noteditmode("ambient")) { hdr.ambient = ambient; calclight(); unsavededits++;});
 
 void calclight()
 {