Bunch of improvements and fixes

Closes: #244 (closed)

This MR is intended to improve Clive by implementing some things "in a more Textual-like way" which will hopefully allow us to bump Textual version easier.

Please look into the commits, but overall:

• Remove the need of cleanup and storing app singleton, use active_app ContextVar instead
• Ged rid of our custom post_message_to_screen and post_message_to_everyone methods
• Don't take additional callback in ConfirmWithPassowrd, use textual dismiss callback

and some other fixes:

• Fix repeat password validation not updating when password is changed
• Fix bug with not displaying public key when alias is wrong on NewKeyAlias screen and form
• Fix bug with checking if NewKeyAliasForm should be skipped
• Fix bug with failing validation when entering NewKeyAliasForm multiple times during onboarding