Fix signal connection nodiscard warnings

[pajlada]

Description

• Fix nodiscard warnings for SplitHeader split connections
• Clean up SplitHeader's mode update code
• SplitHeader: Ensure the layout is only created once
• Clean up SplitInput signal connection handling
• Clean up ReplyThreadPopup signal ignores
• Clean up Application signal connections
• Clean up AccountController signal connections
• Clean up CommandController signal connections
• Clean up NotificationController signal connections
• Clean up Irc2 signal connections
• Change IrcConnection's smartReconnect signal into a function
• Fix signal connections in SelectChannelDialog
• Clean up ChannelView signal connections
• Clean up Split signal connections
• Clean up HighlightingPage signal connections
• Clean up IgnoresPage signal connections
• Clean up ModerationPage signal connections
• Clean up NotificationPage signal connections
• Clean up KeyboardSettingsPage signal connections
• Clean up FiltersPage signal connections
• Clean up GeneralPageView signal connections
• Clean up NicknamesPage signal connections
• Clean up AccountsPage signal connections
• Clean up CommandPage signal connections
• Clean up UserInfoPopup signal connections
• Clean up EmotePopup signal connections
• Clean up TwitchAccountManager signal connections
• Clean up Logging signal connections
• Clean up TwitchIrcServer signal connections
• Mini TwitchIrcServer.cpp refactor :tf:
• Clean up AccountSwitchWidget signal connections when it gets destroyed
• Clean up signal connection in WindowManager
• Clean up InitUpdateButton signal connections
• Clean up some TwitchChannel signal connections
• TwitchChannel/Channel: Convert the messageRemovedFromStart signal into a virtual protected function
• TwitchChannel: Refactor roomIdChanged into a function
• random clang-format reformat on TwitchChannel::threads for some reason

[github-actions]

clang-tidy made some suggestions

[pajlada]