Added world type to the import command and fixed a few things

[Rigner]

This PR changes the following:

• Fixed some String.toLowercase(), String.toUppercase() and String.format(...) not having the locale set to Locale.ROOT. Not setting it would make Java use the current active locale, and could result in weird strings in some specific languages (like in Turkish).
• Fixed a wrong method name in Javadoc, prolly because it has been changed (I just saw it while developing other stuff, thought could be good to fix).
• Made the configuration save every 5 minutes, in case the server crashes or gets killed, to not lose any world data, since it's only saved on clean shutdown atm (We've had a few similar issues in the past, the host crashed and the worlds.yml was weeks old, had to reimport all the worlds we worked on).
• Added a -t parameter to the /worlds import command to specify the world type. Useful when trying to load a world that has been previously created with a void generator, or the END world type as well.

On a side note, I think the -g parameter in /worlds import would be good to rework at some point, right now all values are basically doing nothing except when using CUSTOM generator (see here, the generator argument is not used if not set to CUSTOM).

The -t parameter has been tested using the END one btw.

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• Have you explained what your changes do, and why they add value to BuildSystem?

[thomasmny]

After a quick glance LGTM
Will leave open for a bit longer in case anyone has something they want to point out