fix: Don't use QCompleter prefix filter

[dnsge]

Description

Toggling the "Only search for emote autocompletion at the start of emote names" would require a user to close and reopen a tab for the change to take effect.

The overhaul put the onus of determining completions completely on the Strategy selected. We don't want to limit ourselves to only completing prefixes.

[github-actions]

clang-tidy made some suggestions

[Felanbird]

Bugfix: - Fixed an issue where the setting `Only search for emote autocompletion at the start of emote names` wouldn't disable if it was enabled when the client started. (#4855)

issue could have been* resolved on a per-channel basis without a client restart, but seems like this better explains the bug

[Felanbird]

@pajlada current problem is as follows:

When the client is started with the setting Only search for emote autocompletion at the start of emote names enabled(default), disabling it does not update the completion model, the tab itself must be closed & re-opened or the entire client restarted.
When the client is started with this setting disabled keeping it disabled and enabling it both work exactly as expected

The changes in this PR resolve the first problem.

but, this setting also effects commands see #3440

on 06eb30a (before any input completion overhaul):
Enabled on startup = pains -> pains | popo -> popo
Enabled on startup & disabled | pains -> pains | popo -> popo
Disabled on startup = pains -> Pajapains | popo -> /popout
Disabled on startup & re-enabled = both emotes & commands are properly limited pains -> pains | popo -> popo

on 4d8b623: | tested #4853 as well, no change
Enabled on startup = pains -> pains | popo -> popo
Enabled on startup & disabled | pains -> pains | popo -> popo
Disabled on startup = pains -> Pajapains | popo -> /popout
Disabled on startup & re-enabled = emote completion respects the setting, commands do not pains -> pains | popo -> /popout

on this PR :
Enabled on startup = pains -> pains | popo -> /popout
Enabled on startup & disabled = pains -> Pajapains | popo -> /popout
Disabled on startup = pains -> Pajapains | popo -> /popout
Disabled on startup & enabled = pains -> pains | popo -> /popout

So this PR fixes the issue with commands, but does not fix the problem with commands not respecting the setting, though only partially caused by this PR

Problem likely stems from the way commands are handled in 4d8b623