repo cleanup: remove tcl/tk build infrastructure and scripts, old patches, automated build scripts for pd-extended, old gui plugins, and unnecessary install-time dependencies

update contributor guide, making clear a prototyping language cannot itself be build out of prototypes

port all relevant externals to the new loadbang interface. Unfortunately it has some #defines hidden in g_canvas.h, so we port that macro individually to each affected library.

improvements to the dropdown atom: use scrollbar when menu size exceeds the viewport size. Also, add keybindings for Enter, Escape, Space, arrow keys, and navigating options by matching keypresses to the first character

Clarify the license terms of the package as well as its bundled components,
and place the original license files into the main directory of the source, so
that everybody can find them easily. The source is in Markdown format for
easier editing (LICENSE.md), from which the html file (LICENSE.html) can be
generated with: pandoc LICENSE.md -o LICENSE.html. The html file is now used
in packages/Makefile to generate the License.html file to be included in the
package.

sys_expandpathelems: replacement for @pd_help should be the last element on the path, not the first.

Some gentle reformatting to make the Markdown more standards-compliant, so that it renders correctly on Github.

Simplify the pipeline in sys_initloadpreferences() so that we read the preferences directly using plutil now.

This is done by importing the config data using 'defaults import' in
one go, which is *much* faster than the previous implementation which
invoked the shell to run 'defaults write' for each individual key.