more explicit Toplevel.open_target/close_target; replaced 'context_includes' by 'context' 'includes'; tuned command descriptions;

tuned signature;

optional 'includes' element for long theorem statements; tuned signatures;

basic support for nested contexts including bundles; include multiple bundles; renamed "affirm" back to "assert" (cf. c4492c6bf450 which was motivated by obsolete Alice/ML); tuned signatures;

tuned messages;

basic support for nested local theory targets;

try apple.laf.useScreenMenuBar=false to make menus stay closer to the editor views they belong to -- potentially less confusing for jEdit newcomers;