Patch to allow pre-2.50 Blenders to read newer files.

Patch to allow pre-2.50 Blenders to read newer files.

Since we'll reshuffle a lot in UI code, making new Screens totally
incompatible, this patch saves the Screen chunk in Blender files
with a new identifier (ID_SCRN), causing it to be not read in old
Blender binaries. Pre-2.50 blender already has a facility to recover
from this (it keeps old UI), including for .B.blends (it opens
default simple screen)

For the latter reason, it might be advisable to have the .B.blend
for 2.50+ saved as another name? Then you can use both for while.

(Note: commit is just 3 lines of code, other files are comments I
added for documentation of other stuff)