libgeda: env. vars support in 'component-library-search'

Reported-by: Filippo Micheletti <filippomicheletti@gmail.com>
First commit of discussion document
added wireframe
Add menu hook for the attribute sheet editor.

Added the x_attrib_sheet.c file to contain the connections between
gschem and libgattrib, and created g_key_* and i_callback_* functions
to service it. Finally, added a new menu item in Options to give me something
to test with it.
Create skeleton libgattrib

Add the libgattrib directory to the repository and update
the top level directories in configure.ac and Makefile.am.
Added gitignore files for new libgattrib directories
Add the current GSCHEM_TOPLEVEL to the parameter list for x_attrib_sheet_open()