Unescaping re-added, support limited to schema introspection (#2850 will bring more complete coverage)

@morozov review: According to MySqlPlatform::getListTableColumnsSQL(), the default column is always selected. Removed ??

I know, this is REALLY nasty but that was the only way I've found to
make it work properly.

Fix PSR2 rules : "As per PSR2, the static declaration should come after the visibility declaration."

- fixes platform's COMMENT ON statement support
- implements reverse engineering for column comments
- marks boolean column type commented for distinction

fixes #2182

fixes #1206

fixes #1339
closes #921