Refactor app state into a struct

[hlissner]

What changed:
All emacs-everywhere's buffer-local state has been compressed into an emacs-everywhere-app-info struct, stored in the buffer-local variable emacs-everywhere-current-app (which is given the permanent-local property so its value isn't killed when the major mode is changed).

Also, since the app name is the app's identifier on macOS, it is now used as its emacs-everywhere-app-id, rather than leaving it nil.

Why:

1. It makes it a little easier to juggle this state across the various init and finish hooks, and to clean up (in a later PR).
2. Makes it easier to prevent some type errors and loss of data when changing major modes (possibly fixing Error when trying to write in apps #3).
3. This is the first step toward a larger PR to make emacs-everywhere use Emacs' built-in server machinary for frame life-cycle management, to reduce redundant work and opens up emacs-everywhere for use with browsers or apps with direct EDITOR support, like qutebrowser.