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/plugins/coldcard/cmdline.py

@@ -2,13 +2,15 @@ from electrum.plugin import hook
 from electrum.util import print_msg, raw_input, print_stderr
 from electrum.logging import get_logger
 
+from ..hw_wallet.cmdline import CmdLineHandler
+
 from .coldcard import ColdcardPlugin
 
 
 _logger = get_logger(__name__)
 
 
-class ColdcardCmdLineHandler:
+class ColdcardCmdLineHandler(CmdLineHandler):
 
     def get_passphrase(self, msg, confirm):
         raise NotImplementedError