fixed reversing EW (save as)

when the file name changes, there will no longer be a use-less
save point file. instead the new file is deleted upon rubout.
 * save points are properly created if a file already exists
   with the same name, even though it was not known to SciTECO before
   the save. (e.g. you do save-as to a file that already exists).
 * more effects of the save command can now be rubbed out correctly
changed syntax for long Q-Register names: use [] brackets instead of {}

this breaks many existing scripts, and means you may have to rebuild SciTECO
with ./configure --enable-bootstrap

The syntax of SciTECO might change in backwards-incompatible until
version 1.0 is released.
implemented EC command (execute operating system command) in spawn.cpp

powerful command for filtering a SciTECO buffer through an external
program. It will be described in the sciteco(7) man pages.
The implementation uses an asynchronous background process with
pipes but is platform independant thanks to glib's g_spawn functions,
GIOChannels and event loops.
There are however platform differences in how the operating system
command is interpreted/parsed.
added EG command: pipe from buffer into Q-Register
TAB-completion for EC command

behaves just like ^T in string arguments.
later we might add special Bash-completion support
documented EC and EG commands
prevent assertions when pushing operators without corresponding operands

instead throw an error. The error could theoretically be thrown
earlier instead of only when trying to perform a calculation.

test cases: "++", "+1+", etc.
current_doc_must_undo(): check for undo-necessity when operating on the current document

if the current document is a local q-register from a macro call,
we must not generate undo tokens, since the local documents
are discarded on macro termination.
fixed undoing of initial TECO document changes

 * the TECODocument::undo_edit() function is called before TECODocument::edit()
   and the Scintilla document might still be unitialized
 * fixes e.g. undoing of Xq, ^Uq on registers whose string part
   has not being used before