Use native placeholders

[0tkl]

wx3.0's document:

For the platforms without native hints support (and currently only the MSW port does have it and even there it is only used under Windows Vista and later only), the implementation has several known limitations.

wx3.1+:

Currently implemented natively on Windows (Vista and later only), macOS and GTK+ (3.2 and later).

Now the biggest problem is that there's no wx3.1 or wx3.2 in the official repo of Debian 11 or Ubuntu 20.04/22.04, only wx3.0. Should we turn to depend on wx3.2 from third-party (CodeLite), or we just build wxwidgets subprojects from source during Ubuntu build? An interim alternative is that #if wxCHECK_VERSION(3,1,0) use the native one #else fallback to Placeholder<>. we keep the original Placeholder<> until late-2024 or 2025. If you think the alternative is better I'll push it to here.

BTW I don't know whether SetHint have native wxQt support. Arch Linux have the wxwidgets-qt5 package but i guess there's almost no wxqt users on Arch, because I had tried to build Aegisub with it and found this wxqt package was broken for a long period 😹. Therefore, few people will be affected by this PR even if no native wxQt support.