New feature: Allow customisation of dialog title and button labels

[btzy]

This PR makes it possible for clients to customise the dialog title and button labels. This allows clients to be more specific about the kind of file they want the user to select (e.g. "Open Project") or use different languages.

• Windows: All are customisable (if unspecified they fall back to the OS default), except that the cancel label is not customisable before Windows 7
• macOS / Cocoa: Title and accept button are customisable (if unspecified they fall back to the OS default), but the cancel button is not
• Linux / GTK: All are customisable (if unspecified they are hardcoded)
• Linux / Portal: Title and accept button are customisable (title is hardcoded if unspecified, accept button falls back to the OS default if unspecified)

Resolves #99.

Downgrading CMake for Windows Clang is not not possible or necessary, as windows-latest is now Windows Server 2025.