gui: Make RPCConsole::TabTypes an enum class

Summary:

This change makes the compiler emit a warning/error if a missing enum value is not handled.

This is a backport of Core PR17105

Test Plan:
ninja && src/qt/bitcoin-qt
Test the keyboard shortcuts related to the RPC console tabs.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8159