Reduce use of deprecated functions in filedatedialog.

[cornusandu]

Overview

Reduce use of deprecated functions in filedatedialog.cpp.

Changes

1. Reduce use of deprecated functions [ Primary Focus]

src/dialogs/filedatedialog.cpp was using the setTimeSpec and toTimeSpec deprecated functions. I replaced them in favor of setTimeZone and toTimeZone.

2. Align with modern C++ standards

I replaced const with constexpr where acceptable, as constexpr expresses compile-time intent and already implies constant semantics.

I replaced top-scope char* constants with char[] constants, as constant semantics are better enforced with arrays than pointers.

3. Reduce README noise for users

Non-technical users don't usually need to build the project from source. Replacing that with a collapsible block allows technical users to expand the block, if they wish to build the project from source. This is purely a UX change. Removed upon request.

Notes

The code builds, and I personally didn't get any formatting errors via clang-format.

The commits are intentionally separated. If maintainers prefer, the README and constexpr cleanups can be omitted without affecting the deprecation fix.

[bmatherly]

LGTM

I'm not super excited about the contexpr commit. The change won't make any difference for performance or maintainability. But it adds noise to the change log. So now it will be harder for me to understand the history of a line when I need to do git blame. Not a big deal. And since you already did the work, I think we should take it.

[ddennedy]

Don’t change the readme

[cornusandu]

Don’t change the readme

Done.