restore old \tweak autocomplete

todo: add 2.18 style tweak autocomplete
new-style \tweak autocomplete
fix scheme export, on Windows, HOME is not always set
upd chlog
infrastructure to implement a custom and context-sensitive contextmenu
split requesting the filenames at the cursor from opening them
docstring
right-click on a filename now shows an "Open" action in the context menu
first steps to ly becoming a standalone module and tool
make ly separately packageable and installable
mention ly in INSTALL
touchup python-ly README
a ly command in the root folder that can be run without installing ly or frescobaldi

this is added just to test. It also works when Frescobaldi is installed.
ly command line interface

todo: parse the command itself
more command infrastructure
no error checking in Options.set_variable; do it while parsing
docstring
keep commands and files out of Options
add basic command performing
error handling on opening files