Clean up user-facing strings to facilitate translation

[JesseTG]

This PR makes the following changes in service of a simpler translation process:

• Fixes The use of #ifdefs for some core options complicates translation #151
• Moves most user-facing text to a set of header files

Log messages are not modified; the people who will read them (mostly me) already know English well.

@DisasterMo is the one implementing the scripts needed to manage translation via Crowdin. With that in mind, I have a few questions:

• Is there a way to add translator's notes (even if it's just in the form of a comment)? I don't want to assume that the translators are intimately familiar with melonDS. Plus, there are some fmt arguments that I may need to clarify.
• How do you envision that translated strings besides those defined in core options will be used? I know about retro_core_options_v2_intl, but I'd like to hear your thoughts about looking up the other strings at runtime.
• Once the translation workflow is set up, what should I not change?

[JesseTG]

There are a few things I need to do:

• Add parameters to each (user-facing) format string
• Write translator's notes (just as comments for now)

There are other things I need to do for actual Crowdin integration, but those can come later.