Revert message dialog API

[SaschaCowley]

Reverts PR

Reverts #17304

Issues fixed

Fixes #17560
Fixes #17553

Issues reopened

Reopens #13007
Reopens #12344
Reopens #12353

Reason for revert

This PR broke two important NVDA functions, and there is insufficient time before the Christmas/New Years break to fix the issues, so we are temporarily reverting so that alphas work over the break:

• The CRFT no longer works, as gui.message.MessageDialog's inheritance from ContextHelpMixin was accidentally removed.
• Update checking no longer works, as runScriptModalDialog was modified to increment and decrement _messageBoxCounter, but UpdateAskInstallDialog attempts to restart NVDA before it has been closed, so the update fails as NVDA appears to be in an unsafe state.

Can this PR be reimplemented? If so, what is required for the next attempt

Yes:

• Fix MessageDialog so it inherits from ContextHelpMixin correctly.
• Either:
  • Restore the older, less safe behaviour of runScriptModalDialog; or
  • Re-implement the update check code so it does not try to exit NVDA while a modal dialog is open.