UICommon: Add support for portable.txt on macOS

[OatmealDome]

This PR adds support for portable.txt on macOS. Placing a file named portable.txt next to Dolphin.app will force Dolphin to create and use a user directory in the same folder as the app.

While this is a very un-Mac like behavior (apps aren't supposed to store files next to the app bundle!), it's useful for development.

[Dentomologist]

Would there be a problem if portable.txt and the resulting user folder were inside the bundle? That would allow people to have portable Dolphin in their Applications folder without polluting it.

[OatmealDome]

I consulted with @ryanmcgrath on this, and this is what he had to say:

That’s actually what Slippi effectively did for awhile, and I suggested it in the IRC chat but was shot down

IIRC the logic against it was that it makes testing portable builds annoying since it needs to be done for each build

Personally, I'm fairly neutral on this. Perhaps both methods can be supported?

[OatmealDome]

This technical note states that storing new data within the bundle after codesigning it is not allowed.

(I tried touching a new file inside a codesigned bundle, and macOS seemed to be OK with it. Not sure why, but I think I'd much rather prefer storing the user directory next to the bundle instead of inside of it to be 100% sure we don't accidentally break any future macOS updates.)

[nolrinale]

Because the usual method is to drag and drop the .app into the Applications folder it might pollute the App folder with another folder that might look strange and out of place for mac users specially if they use Launchpad a lot, I honestly think is better to leave the default behavior of keeping the user configs and saves in the Application Support folder like most macOS apps do.

What it can be done is maybe add an "Open Config Folder.." option into the Dolphin app menu bar so the user gets access to the dolphin folder within the library instantly via Finder and can copy/move/replace files if needed for troubleshooting or backup purposes in a more friendly way.

What do you think?

[OatmealDome]

The point of this PR is not to move the User folder. Instead, I'm introducing an option that already exists on Windows and Linux to force the User folder to be placed next to Dolphin.app by creating a portable.txt file. There are various use cases for this, such as running builds for debugging purposes without affecting your actual Dolphin settings used for playing games.

The User folder will continue to remain in ~/Library/Application Support/Dolphin by default.

[nolrinale]

Ah okay since its an optional setting I don't see any problem with it, whoever is gonna deal with this already has the knowledge to troubleshoot any situation that can arise.

[Simonx22]

I've tested this and can confirm that it works as expected on both macOS 12 and Manjaro Linux. Windows wasn't touched by this PR but I've tested that as well.

Code looks good to me too.