Better clipboard support on Linux and Android

[ajusa]

It'd be nice if I could somehow use the clipboard on Android and Linux (those are the two that I tested). I noticed that I cannot copy from a textField, nor can I paste on those two platforms. I was able to do that using the js backend however.

I realize that this may not even be possible on Android due to the usage of SDL, but I think it should be possible on Linux.

[yglukhov]

Yes, I'm planning to migrate to https://github.com/yglukhov/clipboard at some point, but on linux it requires -d:nimxAvoidSdl, and it doesn't support js, android and iOS now.