RTC: Add option to choose between system locale and yyyy-MM-dd HH:mm:ss

[TheTechnician27]

Description of Changes

Adds an option under Emulation > Real-Time Clock called Use System Locale Format. When enabled, the QDateTimeEdit uses the user's QLocale derived from their operating system (current behavior in master). When disabled, the QDateTimeEdit uses the standard format yyyy-MM-dd HH:mm:ss. The option is disabled by default.

Rationale behind Changes

• A standard yyyy-MM-dd HH:mm:ss format is important as a sane default because many users do not include ss in their system's locale. I do, and when I made this feature, I incorrectly assumed this information was always present in the locale but simply truncated in e.g. a user's clock widget. This came up recently when someone trying to do RNG manipulation couldn't access the seconds and had to resort to 1) changing their system locale and rebooting or 2) using the .ini file direclty.
• Allowing the user to use their own locale (current master behavior) is good to retain as people generally prefer or are more comfortable with their own format. Even if the user lacks ss in their format, that level of granularity is useless for some of the biggest use cases for this feature.

Suggested Testing Steps

• Make sure that disabling Manually Set Real-Time Clock also disables the new option Use System Locale Format.
• Make sure that Use System Locale Format 1) starts off as yyyy-MM-dd HH:mm:ss without the option enabled and 2) changes to your system format with it enabled. Keep in mind that your QLocale is likely only changed upon system reboot, so changing your system's locale then trying to see if PCSX2 responds to that is out of our hands.
• Make sure that the tooltip for the new option is sane and doesn't exclude important information.

Did you use AI to help find, test, or implement this issue or feature?

I reset until I found the seed with this PR in it.

[JordanTheToaster]

Spooky.