add structs/code to support alignments and blocks and better hashing from feature context to canvas items.

  the C code yet.

  implementor calls X11::XRemote::block() when receiving a command.
- send_reply automatically unblocks.

I guess the alternative would be to send yourself a command or to emit
an event to act on.  Not sure which is best, but seems like a good idea
to stop people being able to create the race condition.

  setting the app_name, which isn't possible ATM. patience)
- fix what seemed to be a timing issue on linux

- Fixed compiling with -pthread for threaded versions of perl (see comments)

Added comment to help me not break it again!