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