py: Add rules for automated extraction of qstrs from sources.

- add template rule that converts a specified source file into a qstring file

- add special rule for generating a central header that contains all
  extracted/autogenerated strings - defined by QSTR_DEFS_COLLECTED
  variable. Each platform appends a list of sources that may contain
  qstrings into a new build variable: SRC_QSTR. Any autogenerated
  prerequisities are should be appened to SRC_QSTR_AUTO_DEPS variable.

- remove most qstrings from py/qstrdefs, keep only qstrings that
  contain special characters - these cannot be easily detected in the
  sources without additional annotations

- remove most manual qstrdefs, use qstrdef autogen for: py, cc3200,
  stmhal, teensy, unix, windows, pic16bit:

   - remove all micropython generic qstrdefs except for the special strings that contain special characters (e.g. /,+,<,> etc.)
   - remove all port specific qstrdefs except for special strings
   - append sources for qstr generation in platform makefiles (SRC_QSTR)