Full working CEF on GNU/Linux

Hi fellows and friends and Epic Team

I thought I like to share my progress I on an old PR about CEF on GNU/Linux and made it work with all needed keys. Copy, Paste and Cut and the WebBrowserWidget works too.
See here using SlateViewer

Ok, now the WebBrowserWidget works too. See here:

Ok, finished. Made a PR for this. Here you can test if you like

Read the description about the issues please. If you want to try this PR do not forget to compile UnrealCEFSubprocess.