fix up Windows-specific build script to just rsync the pd/doc directory. At some point we need to use makefile.in for Windows just like Linux and OSX do...

keep midr-mjpeg.mov in pd/doc/media, but remove all the copies of external media files. There aren't any references to those copied files in any of the docs, so they are useless. This means the root "doc" directory is now gone.

remove patches from "messageoddness" directory that were either fixed earlier in Pd-l2ork or are misleading/wrong

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.