Add support for borderless Windows

[Flamefire]

Continuation of #1602 to add a 3rd mode:

• Fullscreen
• Windowed
• Borderless window

This removes the whole window frame making it non-resizeable:

Closes #1602 Fixes #1512

In the options screen I added a similar drop-down.

As an extension I would change "Fullscreen resolution" to "Window size" when selecting a windowed option in both settings. There all resolution options and the current window size will be added. Shall I? @Spikeone

[Spikeone]

Sounds good to me - just some question, I never thought about borderless window (Although this is probably what I'd like to have anyway) but wouldn't this be somewhat confusing for people since most would think its fullscreen windowed (as most modern game name it) but we don't have such option at all?

[Flamefire]

My concern is rather: Without a title bar you cannot move it and it will always be centered. Not sure this suits everyone but you can't have that anyway.

fullscreen windowed (as most modern game name it) but we don't have such option at all?

The closest would be to set the size accordingly.
Additionally, if fullscreen isn't possible I implemented a fallback to a fullscreen, borderless window

That being said we can add a 4th option "Fullscreen windowed" (or maybe "windowed fullscreen" or "fullscreen window"?) to automatically set the size to the desktop size. IIRC SDL has even a flag for that, something called FULLSCREEN_DESKTOP or so.

[Spikeone]

Oh, so the borderless window is the one that is not resizable? I honestly thought we'd just ignore the resize event and add an option to ignore that event

[Flamefire]

Having no border makes it clear that the window cannot be resized as there is nothing to do that.
For the normal window we actually set a flag to make it resizable. The original PR #1602 was using a bitset so we can represent that in our settings too. Downside of course are some combinations like "resizable fullscreen" make no sense. I'd ignore that though.

I see it rather as an UI issue: Now we have 3 clear display modes. How/Where would be disable window resizing? Or shall we have a "fixed size window" option there?

[Spikeone]

Well, what I was thinking about in general:

• Fullscreen
• Fullscreen Borderless Windowed
• Windowed

Those are the 3 modes I'd expect. Then I'd just love to have an additional option to:

• Enable / Disable Resize window

Which would only apply to windowed mode. I still want to move the window around, but I don't want to resize it by accident. That was my initial intention when I asked @falbrechtskirchinger for this :)

[Flamefire]

Hm, back to a bitset for the option then:

• Fullscreen
• Windowed
• Resizeable

Fullscreen | Windowed would then be a borderless window and Resizeable only specifically handled for non-Fullscreen, ignoring it for fullscreen option

[Spikeone]

Hm, back to a bitset for the option then:

* Fullscreen

* Windowed

* Resizeable

Fullscreen | Windowed would then be a borderless window and Resizeable only specifically handled for non-Fullscreen, ignoring it for fullscreen option

If I don't missunderstand this - yes. (Honestly, as long as I can play windowed, move the window and prevent it from resizing by accident, I'm happy)