Case sensitivity issues in handing IBusConfig value-changed

[bjj]

https://github.com/phuang/ibus-pinyin/blob/4950900e3acc6cf0741962179e2b2b924964ae4f/src/PYConfig.cc#L289

IBusConfig sections are case-insensitive (inherited from dconf?) and value-changed always reports section names in all lowercase.

The linked line of code does a case-sensitive compare of the "section", and it believes its section is "engine/Pinyin" so it ignores value-changed events.