Add PBKDF2 customization

[Macha-orange]

• Add a "PBKDF2 rounds" button inside "Show Entropy Details" with values : 2048, 4096, 8192, 16384, 32768, custom.
• Add hidden button for custom iterations. Accept number.
• Add a Warning message above BIP39 seed when using anything other than 2048 PBKDF2 iterations.
• Add a "PBKDF2" section in the bottom for detail.

[iancoleman]

I'm pretty conflicted about this feature, but ultimately I think it has a good use for advanced users so it's worth including. Thanks for the PR.

Some notes for myself

• add tests for this
• consider position of warning message - right below the select or just above the address table? Probably right below the select. Not sure where is the most 'obvious' place to put it. Consider these steps:
  • show entropy details
  • change pbkdf2
  • hide entropy details
  • newly generated mnemonics still use the custom pbkdf2 setting, so maybe the current place for the warning just above the mnemonic is most appropriate?