hw wallets: introduce HardwareHandlerBase

previously, client.handler was sometimes
- an InstallWizard
- a QtHandlerBase where win was an ElectrumWindow
- a QtHandlerBase where win was an InstallWizard
- a CmdLineHandler

That's just too much dynamic untyped undocumented polymorphism...
Now it will never be an InstallWizard (replaced with QtHandlerBase where win is an InstallWizard),
and now in all cases client.handler is an instance of HardwareHandlerBase, yay.

related: #6063

electrum/base_wizard.py

@@ -535,6 +535,7 @@ class BaseWizard(Logger):
         if self.wallet_type == 'standard' and isinstance(self.keystores[0], Hardware_KeyStore):
             # offer encrypting with a pw derived from the hw device
             k = self.keystores[0]  # type: Hardware_KeyStore
+            assert isinstance(self.plugin, HW_PluginBase)
             try:
                 k.handler = self.plugin.create_handler(self)
                 password = k.get_password_for_storage_encryption()