Qt: Add button to main menu's view tab to allow GUI to enter fullscreen.

[SDreadnoug2]

Description of Changes

Fulfills the request for a feature in #14405 which asks to add the option to enter fullscreen from the main menu bar, rather than having to add MainWindowFullscreen = true to the PSCX2.ini file.
In the view tab of the main menu bar, I've added a Fullscreen(GUI) check box, which on clicking enters fullscreen, and on clicking again exits fullscreen.

Rationale behind Changes

Currently users have to edit their PCSX2.ini file, adding MainWindowFullscreen = true. Whilst this works, it requires a restart of the program/knowing you're supposed to do that. This simplifies things to a check box.

Suggested Testing Steps

1. Open PCSX2 and open the View tab.
2. Click Fullscreen (GUI)
3. The program should enter Fullscreen mode.
4. Click Fullscreen (GUI) again, the program should exit fullscreen mode.
5. Opening and closing the program in either mode keeps the program in that state.

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

no

[github-actions]

Thank you for submitting a contribution to PCSX2

As this is your first pull request, please be aware of the contributing guidelines.

Additionally, as per recent changes in GitHub Actions, your pull request will need to be approved by a maintainer before GitHub Actions can run against it. You can find more information about this change here.

Please be patient until this happens. In the meantime if you'd like to confirm the builds are passing, you have the option of opening a PR on your own fork, just make sure your fork's master branch is up to date!

[chaoticgd]

You can call Host::CommitBaseSettingChanges(); here to save the setting.

Also use git-clang-format (or just regular clang-format if there aren't too many unrelated changes) to format your changes.

[chaoticgd]

I'm worried this has the potential to cause confusion. Like if someone tries to enter fullscreen normally, but clicks this option accidentally. Then they'd be left wondering why exiting fullscreen normally doesn't actually exit fullscreen.

[Pesa]

Does this really need a menu item? Too many options can cause confusion, UI clutter, and cognitive overload for users. Especially in this case where the two menu items are very similarly named (and "GUI" is just confusing, the game window is also a GUI technically).

Moreover, I didn't really see a justification for adding this option.

• [Feature Request]: Make it possible to open the main window in fullscreen from the GUI #14405 doesn't provide a rationale other than "there is an ini setting for it".
• Qt: Add MainWindowFullscreen to ini for human readable fullscreen override #14401 added the aforementioned fullscreen setting, referring to [Feature request]: MainWindowMaximized, and MainWindowFullscreen boolean setting #13249 for the rationale.
• However, [Feature request]: MainWindowMaximized, and MainWindowFullscreen boolean setting #13249 doesn't provide a justification for MainWindowFullscreen, only for MainWindowMaximized.

My suggestion, if you want to expose this setting in the UI, is to change the existing "Start fullscreen" checkbox in Interface settings into a combobox with 4 choices: Never, Game, Main Window, Both (exact wording TBD). Or alternatively add a second checkbox there to fullscreen the main window.

Either way, please at least consider hiding this new menu item on macOS. On macOS, every app has a built-in menu item "Enter Full Screen" added by the OS, so this new item is redundant.