Move desktop-related code to "desktop" subfolder (#872)

Summary:
Pull Request resolved: https://github.com/facebook/flipper/pull/872
Move all the JS code related to desktop app to "desktop" subfolder.

The structure of "desktop" folder:
- `src` - JS code of Flipper desktop app executing in Electron Renderer (Chrome) process. This folder also contains all the Flipper plugins in subfolder "src/plugins".
- `static` - JS code of Flipper desktop app bootstrapping executing in Electron Main (Node.js) process
- `pkg` - Flipper packaging lib and CLI tool
- `doctor` - Flipper diagnostics lib and CLI tool
- `scripts` - Build scripts for Flipper desktop app
- `headless` - Headless version of Flipper app
- `headless-tests` - Integration tests running agains Flipper headless version

Reviewed By: passy

Differential Revision: D20249304

fbshipit-source-id: 9a51c63b51b92b758a02fc8ebf7d3d116770efe9

desktop/patches/adbkit-logcat+2.0.1.patch

@@ -0,0 +1,13 @@
+diff --git a/node_modules/adbkit-logcat/lib/logcat/parser/binary.js b/node_modules/adbkit-logcat/lib/logcat/parser/binary.js
+index 12fa8ec..e122933 100644
+--- a/node_modules/adbkit-logcat/lib/logcat/parser/binary.js
++++ b/node_modules/adbkit-logcat/lib/logcat/parser/binary.js
+@@ -10,7 +10,7 @@ const HEADER_SIZE_MAX = 100
+ class Binary extends EventEmitter {
+   constructor(options) {
+     super(options)
+-    this.buffer = new Buffer(0)
++    this.buffer = Buffer.alloc(0)
+   }
+ 
+   parse(chunk) {