Add scintilla constants

[scriptum]

scintilla_send_message accepts a magic number in first argument - message id but GeanyPy doesn't provide any human-readable constants for that. There are another issue with send_message (see #32).

It's easy to generate automatically: see https://github.com/gejzenbug/scintilla-python-constants

[elextr]

The link above does not show the generator script but it would be easy to make one, note that it will have to be re-generated for each version of Scintilla since the Scintilla files are also autogenerated so you don't know what might change.

Note also the link above is GPLv3 and Geanypy is GPLv2+.

[scriptum]

sed -rn '/^#.*[0-9]$/s/#define\s(.*)\s/\1 = /p' SciLexer.h Scintilla.h

[elextr]

@b4n, any thoughts on how this is kept in sync with each Scintilla update, can you push/PR this as part of the Scintilla update process?

[scriptum]

how this is kept in sync with each Scintilla update

Adding this simple line into makefile probably would solve the problem (automatically generated source).