The big segmented buffer is freaking.
One segment is permanently used as heap
One segment is used for the file database and the listing of file names in the menu
14 segments are used for playback buffer

Problem: File Read operations are given by the OS
 & they may call a thread switch
 & they write on the big buff
 & we don't want to lock the whole menu just for file copying

Solution:
  Change the segment to 1 only as long as we are within the menu.
  This is done with banking_displayuse(); and banking_displayrelease();
  The banking_displayuse() may not be called twice in a row
  (no there isn't a stack implemented).

The first menu_redraw() does not need this, because:
  a) He does not use the banking *and*
  b) a redraw has no risks to call menu_action where the functions above are used.

To make it even more difficult: The fileselect.c and filemanager.c are
playing their part too, needing the same segment as the menu_ functions.
The fileselect may use the banking_displayuse() function for this purpose,
  It can only be called if banking_displayrelease() got called first.
  But thread switching in Nut/OS functions must be considered.


