refactor: Use override instead of virtual where possible

[pajlada]

Description

This PR fixes most warnings from the clang-tidy cppcoreguidelines-explicit-virtualfunctions check

1. For base classes implementing virtual functions, just use virtual
2. For derived classes overriding a base class virtual function, just use override since override implies virtual

Part of #4915

Unrelated change(s)

Irc2: Add some newlines to let the anon namespace breathe
LambdaRunnable: Initialize action_ in member initializer list
IrcChannel: Made the class final
TwitchChannel: Made the class final

How to review

Most changes are just removing virtual & the reformatting that comes with it.
TwitchChannel had 3 virtual functions that were never overwritten, refresh{BTTV,FFZ,SevenTV}ChannelEmotes. I removed virtual complete there.
Made LambdaRunnable's run function override - it was not override or virtual before, but run is most likely meant to override QRunnable's run function.
Made GeneralPage's filterElements function override
ComboBoxItemDelegate now uses override instead of virtual, since they override functions from QStyledItemDelegate

[pajlada]

I have looked through all changes in this PR and they look good to me - I'll leave it up for a bit to give people time to look through it before I merge it in.

[github-actions]

clang-tidy made some suggestions

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[github-actions]

clang-tidy made some suggestions