Release 0.4.4: Graphic objects, tagged objects, Undo and LastArgs

Notes:

• This release switches the DM42 build to "optimize for size", because
  we ran out of program space for an "optimize for speed" build.
  The DM32 build is still optimized for speed.
• The release is not really "well rounded". Notably, graphical
  rendering of stack objects is active, but not complete. Everything
  is still rendered from text, and there is no caching yet.
• The package is now consistently called DB50X for DM32 and DB48X for DM42.
  In the long run, the DB50X is expected to be more full-featured or slightly
  faster everything being equal, due to limitations in DM42 program space that
  are fast approaching.

New features:

• Graphic objects, aka grob(#30)
• Graphic operations: GOr, GXor, GAnd (#383)
• Method for graphical rendering of objects (#47)
• Tagged objects, dtag and ->tag commands (#21)
• LastArg (#379, #55)
• Undo, i.e. last stack (#380, #55)
• LastX (#384, #55)

Bug fixes:

• Fix latent memory corruption bug in text constructor
• Fix out of memory error reporting while building text objects
• blitter: Fix clipping when src and dest don't have same x direction

Enhancements:

• Use the help method to select help topic (#378)
• Graphical rendering of stack (#382)
• Simulator: Fix program exit with EXIT key after another key (#356)
• Simulator: Set 40K as default memory
• Out of program space: Switch to -Oz on DM42
• Generate different help files for DB48X and DB50X

Signed-off-by: Christophe de Dinechin christophe@dinechin.org