linux-v4l2: add support for controls

[grimmy]

Video4Linux exposes controls for capture devices. This commit adds
those controls to the properties window for v4l2-input devices.

Few things to mention here. This dynamically adds properties and as far as I can tell, this isn't something that's been done in OBS before, so it's kind of hacky. Also, right now properties don't get cleared if you switch devices which leads to all sorts of issues. I'm working on that, but it's going to be hacky too, requiring a hidden setting to the device or something.

This also means that I can't mark the strings for translation, as they're coming from the v4l driver. There might be a way to load in the v4l get text domain and do it that way, but I haven't investigated that yet.

[grimmy]

Additional controls from my HDMI capture card

Additional controls from my Webcam (i grayed out the image fwiw). Note that the scroll bar shows that there are more controls than displayed.

[jp9000]

Wasn't worth. Been having too many problems with this code.

[grimmy]

I asked for input months ago, and it was merged without ever giving that input. That is, how to handle dynamic controls. It looked like it could maybe be delete with an API that I can't recall at the moment, but yeah, if there's a way I'm totally down to finishing this up...

[kkartaltepe]

You can see an example of a seemingly perfectly functional v4l2 controls implementation in guvcview (GPLv3)

https://sourceforge.net/p/guvcview/git-master/ci/master/tree/gview_v4l2core/v4l2_controls.c

[grimmy]

The problem with this is that when you change a device in the obs drop down it queries the new device and add the properties for it. This is problematic because the old device and new device may share property names so everything becomes a jumbled mess.

The other alternative that I was asking about, is if it'd make more sense to have a separate window for the properties like there is on windows (i think?).