Seperate LibTable class from check_lib_table functionality

[seppestas]

This makes re-using the LibTable class a bit easier and logical. It also remove schematic lib specific code from LibTable making it re-usable for footprint libraries.

Could be used to e.g check custom options in LibTables. I plan to use it together with the functionality of #297 to only check "approved" symbols in a symbol library by having a CSV "SYMBOLS" field in the library options. This makes something like this possible:

# Check "enabled" symbols of all symbol libraries in a symbol library table for KLC compliance
import subprocess
import re
from importlib import import_module
LibTable = import_module("kicad-library-utils.lib_table").LibTable

lib_table = LibTable('sym_lib_table')

for library in lib_table.entries:
	re_symbols= re.search(r'SYMBOLS=([^ ]+)', library['opt'])
	checklib_args = []
	if re_symbols:
		symbols = re_symbols.groups()[0]
		checklib_args.extend(['-c', symbols])

	checklib_args.append(library['uri'].replace('${KICAD_SYMBOL_DIR}', '../symbols'))
	# TODO: make checklib work as python module
	subprocess.call(['./kicad-library-utils/schlib/checklib.py'] + checklib_args)

This could also be used to add an option to ignore certain rules for a symbol library, see #296.

[poeschlr]

thanks