Migrate plLocalization to `ST::string`

[dgelessus]

And remove some unused and/or obsolete methods.

The changes touch a few parts of the Max plugin, which I cannot test properly, as usual.

This class could use some further cleanup too:

• The language numbers are typed weirdly - they're often cast back and forth between plLocalization::Language and plain int, sometimes offset by 1 for mysterious reasons.
• kNumLanguages is one higher than GetNumLocales(), presumably so that kJapanese (the last language in the enum) isn't shown to players.
• The LocalToString/StringToLocal mechanism seems to be obsolete - everything now uses pfLocalizationDataMgr instead of storing translations in the PRPs. pfGUITextBoxMod is the only place still using this old mechanism, but even there it's effectively dead. PRPs still have $-separated strings stored in text boxes, but have all translations blank except English, relying on pfLocalizationDataMgr for the actual translations.

[zrax]

Looks like plInputMap.cpp is missing an <algorithm> include, exposed by header include changes here

[dgelessus]

Ah, thanks for the heads-up. This wasn't noticeable on Windows, because hsWindows.h includes <algorithm>. Pushed a fix to #1358 - to avoid causing merge conflicts, because that PR already touches the plInputMap.cpp includes.

[Hoikas]

I think this needs to be rebased to fix the two CI failures.