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/water.cpp

@@ -4,7 +4,7 @@ int wx1, wy1, wx2, wy2;
 float wsx1, wsy1, wsx2, wsy2;
 
 VARP(watersubdiv, 1, 4, 64);
-VARF(waterlevel, -128, -128, 127, if(!noteditmode("waterlevel")) { hdr.waterlevel = waterlevel; unsavededits = 1; });
+VARF(waterlevel, -128, -128, 127, if(!noteditmode("waterlevel")) { hdr.waterlevel = waterlevel; unsavededits++; });
 
 void setwatercolor(const char *r, const char *g, const char *b, const char *a)
 {
@@ -22,7 +22,7 @@ void setwatercolor(const char *r, const char *g, const char *b, const char *a)
         hdr.watercolor[2] = 20;
         hdr.watercolor[3] = 178;
     }
-    unsavededits = 1;
+    if(editmode) unsavededits++;
 }
 
 COMMANDN(watercolour, setwatercolor, "ssss");