map file addons: embedded config file; persistent undo/redo data

This commit establishes an extensible way of storing additional data in
map file headers. Those header extras can be permanent, like the embedded
config file or be used only once after map load like the undo/redo data.

Additional extra data can be easily added and will not break backward
compatibility, so this map format should hold up for quite some time.

To make things easier for mappers, maps can now be saved without invoking
the various optimisations. That, in combination with the stored undo data,
means, that editing a map can be picked up at exactly the same state, as
the map was saved in.

Maps will automatically be written in the appropriate fashion, depending
on the situation:
 1) for sendmap: no optimisation, no undo data
 2) savemap during editing: no optimisation, with undo data
 3) savemap when not editing: optimised, no undo data
 4) savemapoptimised: optimised, no undo data
(For savemap to save different maps during editing, the variable
 preserveundosonsave has to be "1". It is "0" by default.)

All of the above map files can be played without problems - they only
differ in file size. (However, maps with undo data may be too big to send)

The new command "embedconfigfile" will load the config file, include it in
the map header and rename the original text file.

"extractconfigfile" will write an embedded config file to a separate file
and remove it from the map header.

Embedded config files take precedence over regular config files and
therefore make it harder for cheaters to play with modified config files.

further changes: cleaned up some debugging code; more ASSERTs
multiple map buffers for map editing: "xmaps"

An xmap holds everything related to map editing at a certain moment: it is
a map editing snapshot. It contains the name, the header, the geometry,
the entities, the extra headers, the undo/redo data and the cameraposition
from the moment, the snapshot was taken.

Xmaps are held in memory and, if persistentxmaps is set to "1", are saved
at game end and restored at game start.

xmap_list
  lists all xmaps currently in memory

xmap_store <nickname>
  create a snapshot and store it under the given nickname
  (nicknames have to qualify as valid filenames)

xmap_restore <nickname>
  restores the named snapshot. Also creates a backup snapshot of the
  current editing data.

xmap_restore
  restores the last automatic backup snapshot. Automatic snapshots are
  taken, a) before a snapshot is restored, b) before a new map is loaded
  (if there were unsaved edits on the map) and c) when the game ends (if
  there were unsaved edits).

xmap_delete <nickname>
  deletes the named snapshot.

Unsaved edits are flagged in the HUD with an "U" in the lower right
corner of the screen.

The files with the persistent xmaps are stored in mapediting/xmaps. After
they were restored at game start, the files are moved to
mapediting/xmaps/backups. Xmap files are cubescript files.