Fixed an issue when saving the flame colors in CustomData.

- Implemented the partial overwriting of CustomData, so that only the flame color data is overwritten not the whole contents of CustomData
- Renamed the "Override Flame Colors" checkbox to "Lock Flame Colors" because it explains its function better.

Color information is saved in CustomData, so it's kept between reloads and even in blueprints. It replaces the contents of CustomData for now, but I am planning to make it bit less aggressive and just replacing its own data in case someone wants to store some other things in CustomData as well.