Some native clipboard support for some platforms

I was hoping SDL2 would get this functionality eventually, but nope, proper clipboard support is staged for SDL3, which we're not going to see much of for at least a few more months. This will have to do for 98.0. The feature can be disabled at runtime from powder.pref.

Implementation status:

 - windows (via winapi): has the most friendly api so of course the implementation is flawless and uses every available optimization >_>
 - macos (via cocoa): I'm bad at cocoa so this is only as good as absolutely necessary; TODO: on-demand rendering
 - x11 (via xclip): I am NOT implementing icccm2; TODO: remove reliance on external tools
 - wayland (via wl-clipboard): oh god wayland oh why, you're almost as bad as x11; TODO: remove reliance on external tools
 - android: TODO; is there even a point?
 - emscripten: TODO; the tricky bit is that in a browser we can only get clipboard data when the user is giving it to us, so this will require some JS hackery that I'm not mentally prepared for right now; also I think the supported content types are very limited and you can't just define your own

x11 and wayland support are handled by a common backend which delegates clipboard management to xclip-like external programs, such as xclip itself or wl-clipboard, and can load custom command line templates from powder.pref for use with other such programs.

cross-examples/macaa64.ini

@@ -1,6 +1,7 @@
 [binaries]
 c = [ 'clang', '-arch', 'arm64' ]
 cpp = [ 'clang++', '-arch', 'arm64' ]
+objcpp = [ 'clang++', '-arch', 'arm64' ]
 strip = 'strip'
 
 [host_machine]