Fix several inspections

[FWDekker]

I ran some inspections on the code base and found the following potential issues:

• InsertAction uses a form of runWriteCommandAction which is annotated as @TestOnly. I should use the one with a different signature, where I also specify the action's name.
• radioButtonGroupProp in AssertJSwingProperties is never used. Remove it.
• DefaultTemplateActionLoader should (apparently) not be public (but internal).
• Various fields in plugin.xml should support i18n. (Re-run the inspection to see which ones I mean.)
• StringScheme.Companion#DEFAULT_CAPITALIZATION should be = instead of get() =.
• Do I still need the com.vdurmont:emoji-java dependency? (Adds tests for this!) (Edit: Emoji are not explicitly supported, but also don't need explicit support like they used to.)
• If I'm looking at dependencies anyway, document once and for all how it works with the kotlin-reflect dependency.
• Consider adding explicit return types for most methods (enable this as an inspection where possible). (In Detekt?) Don't fix this right away everywhere, that'd be nasty. Make it gradual. Nope, looks ugly in many cases. I'll decide on a case-by-case basis.
• Use ProjectActivity instead of StartupActivity to notify users. (Test this in various versions!) (Edit: Tested. Both implementations work in both 2023.1 and 2023.3. I'll stick with StartupActivity because that's the more modern one.)

Also, I can

• add a message to the automatic error reports indicating that users can subscribe to the issue to get updates, and add information on how they can delete the message, and ensure the privacy notice in the error reporter itself mentions GitHub.
• Also, change the GitHubScrambler to use actual scrambling instead of overly fancy encryption. (Edit: Not that important, really.)