Add SPACE, DEL icons to Keyboard; various UI consistency tweaks

[kdmukai]

Description

See affected screenshot changes: SeedSigner/seedsigner-screenshots#5

Main purpose:

• Adds the new SPACE and DEL icons from Messaging Components - Colors and Icons #605 to Keyboard. Slight refactor in there required.
• Requires a minor bugfix on the icon constants mappings.

UI fixes:

• We had never used the ERROR icon (we were using the WARNING exclamation in red instead).

UI consistency tweaks:

• Slightly opinionated edits for when to use ERROR vs WARNING and red vs yellow.
• General principles applied:
  • Red = Extreme danger (displaying mnemonic, PSBT addr verification failed, etc)
  • Red = System error, bug, showstopper
  • Yellow = Warning; be aware but don't scare the user.
  • Yellow = Incompatibility (e.g. unknown QR format). Yes, something went wrong, but don't scare the user.
• Judgment call: Signing PSBT didn't produce a valid signature: I used WARNING in yellow.
  • Rationale: maybe they already signed the multisig PSBT with this key. That's not a reason to panic. Or they opted to try to sign with a key that we didn't think could sign in the first place (has the "(?)" on the selection screen).

Minor misc:

• Keyboard: Use our CHEVRON_LEFT / CHEVRON_RIGHT for cursor navigation.
• I/O Test Screen: Use our icons instead of FontAwesome.
• Remove legacy load_icon() function which was an early ~v0.5.0 relic.
• Minor spacing fix at the bottom edge of ButtonListScreen where is_bottom_list=True with more than one button.
• Render Dice icons to fill the Keyboard key size.
• Change to text when multisig self-transfer addr can't be verified by descriptor; edited to fit better in available space. Text change affects translations (messages.pot).
• Applied "Suspicious PSBT" consistently across single sig and multisig.
• Screenshot generator tweaked to generate same results as the ScanView error when it scans an unsupported QR format.

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This pull request is categorized as a:

• UI cleanup

Checklist

• I’ve run pytest and made sure all unit tests pass before sumbitting the PR

If you modified or added functionality/workflow, did you add new unit tests?

• N/A

I have tested this PR on the following platforms/os:

• Raspberry Pi OS Manual Build
• SeedSigner OS on a Pi0/Pi0W board
• Other

[jdlcdl]

As of 5c6c99b

This is working for me. Have reviewed code changes, all reasonable to me.
Have tested a little, but will test more, on pi0 built by seedsigner-os pr-83
Have carefully reviewed screenshots in english, compared to merges made mid-last week, all as expected given the code changes.