Add experimental support for MS-Windows with occasional bugs

[hanwenguo]

I've tried to add MS-Windows support to emacs-everywhere and here is the outcome. It doesn't work as a charm: it works most of the time, however bugs take place randomly (there are possible solutions). But something is better than nothing.

The support is mainly introduced by defining paste/focus/copy/app-info commands, considering missing MS-Windows cases and making some modifications to emacs-everywhere, emacs-everywhere-insert-selection in order to handle some MS-Windows specific problems without affecting their original functions.

Some drawbacks:

1. Heavy usage of calls to Win32 API through P/Invoke by PowerShell commands. The reason is that, as there lacks utilities like xdotool and AppleScript on MS-Windows, this is the only way to manipulate windows/frames without dependence on external tools. One who can withstand external tools or files can create wrapper for the commands with PowerShell script files or some simple C# programs by their own.
2. Not universal. The commands simulate C-c and C-v to copy and paste selections between target window and Emacs, so whether it get the right selection depends on how the target window handles these keystrokes. For example, it cannot be used with TeXmacs, which uses Emacs-style keymap so C-c doesn't copy and C-v doesn't paste. There might be other approaches, such as sending WM_COPY message.
3. Instability. It cannot yank or do paste (though clipboard is actually changed) sometimes, this happens randomly. Possible solutions include telling the user to copy/paste manually before/after emacs-everywhere, which needs only two more keystrokes and will work like a charm.
4. Slow speed. I don't know whether it is only my PC or generally the MS-Windows system that makes creating frames and calling processes very slow, yet acceptable.

[tecosaur]

Thanks! I'm open to merging this, but I've got a few comments on your coding style.

[hanwenguo]

I just pushed a new commit to improve the coding style, please give it a review. 😄

[tecosaur]

Looking much better :) I'll give this a better look this evening

[tecosaur]

Apparently that evening is now 😅, looks good — all that's missing is some docstrings 🙂

[hanwenguo]

Done! Please review it. 😄

[tecosaur]

Sorry for the delay, and thanks. Merged (via Magit not GitHub)!