Qt: GBA QoL improvements

[Bonta0]

This adds features frequently requested by users to the GBA windows

• Borderless mode
• Always on top
• Interframe blending (see https://mgba.io/2020/01/21/mgba-0.8.0/#interframe-blending)
  
  This also exposes Core features better to make a couple conditions in the context menu more accurate

[JMC47]

@dolphin-emu-bot rebuild

[JMC47]

Everything is tested to work. Tested Animal Crossing and Nintendo Puzzle Collection for Interframe blending. Tested fullscreen shenanigans for the always on top and borderless GBA.

[dolphin-emu-bot]

FifoCI detected that this change impacts graphical rendering. Here are the behavior differences detected by the system:

• soniccolors-mm on uberogl-lin-radeon: diff

_{^{automated-fifoci-reporter}}

[WeedyWeedSmoker]

This commit breaks compatibility with macOS 10.13 (still works on the newer macOS 11 version, didn't test other macOSes…)

@Bonta0, @JMC47

[JMC47]

I can update the FAQ to change the minimum requirements.

[JosJuice]

But I can't see anything in this PR that would cause macOS 10.13 to break... I don't think you should update the minimum requirements until the reason why it broke is understood.

[WeedyWeedSmoker]

@JMC47 @JosJuice Let me test again, this specific commit now seems to work fine on 10.13, I'm checking which commit actually broke it, this commit is indeed important!

Edit : It seems to be the commit right after this one which crashes macOS 10.13 with a Segfault…

[JMC47]

I just meant that as a nuclear option, we can raise the minimum requirements. Obviously if it's something silly or unintended, we can try to fix it.

[WeedyWeedSmoker]

@JMC47 @JosJuice 5.0-14748 crashes, while 5.0-14746 works fine… The culprit is between this commit (3e04cb6) and 7fe97b2, I will try to compile those in-between commits to try and find the real culprit, while 46e331d seems to be a good candidate for a crash on macOS 10.13 somehow…

[WeedyWeedSmoker]

@JMC47 @JosJuice I can't manage to compile Dolphin on macOS, finally got my Qt5 detected, but then got errors with <optional>, trying this : https://github.com/youknowone/xcode-libcxx but brew got certificate errors trying to download a required part of llvm…

Do you have a way to compile the aforementioned commits for mac ?

(namely 46e331d, 9a33fd3 (certainly not related) and c9a4021 …)