Update supported CEF version, and a bunch of other changes

[em-tg]

I tried using this repo as a jumping-off point for a project a few years ago, and am thankful it exists. It was a little difficult to get started with it, though, since it didn't work with the latest version of CEF, and had a number of other issues. Since it doesn't seem to have received any updates since then, I thought I'd submit a PR so others don't have to struggle as much as I did.

This PR updates to the latest CEF version, fixes a number of issues, implements "true" reference counting, and "cleans" the C a bit. Some of the changes are just random things I thought would be a good idea, so feel free to edit and strip out the bits you don't want (or tell me to do so).

List of changes:

• Update both the Linux and Windows builds to support CEF 123.0.7
• Make extraction of CEF an automated part of the build process
• drop support for gtk2
  • I couldn't get it working locally, but I didn't spend much time on it and there may have been something I was missing
• remove custom gtk window creation code
  • I think this was working around a bug in CEF that's no longer in the latest version
• Fix handling of reference counted objects so the program no longer crashes on exit
  • This includes removing a bunch of the stub handlers from cef_client.h and cef_app.h, since I didn't want to go through adding release calls all over the place in functions that weren't being used
• Implement "real" reference counting in addition to the "dummy" one
  • The reference counting implementation is fully generic (using the preprocessor), but I think it's helpful to demonstrate that reference counting isn't technically necessary for most handler implementations, so I just implemented a reference-counting example for one of them, and left the others to continue using the "fake" reference counting.
• Smaller changes:
  • Handle the error returns from cef_initialize and cef_browser_host_create_browser
  • Create cef_strings directly from UTF-16 string literals rather than using ASCII literals and converting
  • Avoid using pointer casts when they are not necessary
  • Eliminate the global client_handler_t and use container_of to extract it from cef_client
  • Use C99 designated initializers in more places
  • Prefer sizeof val to sizeof(type)
  • Add an implementation of cef_load_handler to demonstrate reference counting
• On Windows:
  • Most of the above
    • NOTE: Extracting cef*.tar.bz2 using the build scripts requires 7zip. Windows has tar but it can't handle .bz2 :P
  • Add support for MSVC (Tested on a computer with VS 2022, although it should work with any recent version)

[em-tg]

cefcapi appears to be affected by chromiumembedded/cef#3765

[em-tg]

Should be good to go, now