Fixes #13625

The terminal should be a lot faster at processing output now thanks to
sourcelair/xterm.js#422

Also of note: sourcelair/xterm.js#417: changed escape sequence for alt-arrow to work on bash on os x

This adds support for building Flatpak application bundles, which should be
runnable on any Linux distribution with Flatpak support. See the page at
http://flatpak.org/getting.html

New Gulp tasks are added to prepare a directory with the needed files
(vscode-linux-${arch}-prepare-flatpak), assembling the Flatpak bundles
(vscode-linux-${arch}-flatpak), and for cleaning the build directories
(clean-vscode-linux-${arch}-flatpak). This mimics how the Debian and RPM
package creation works. Hence, building an application bundle is done with:

  $ gulp vscode-linux-x64-flatpak
  [...]
  $ ls *.flatpak
  com.visualstudio.code.oss-x86_64.flatpak

Installing and running the application is achieved with:

  $ flatpak --user install --bundle com.visualstudio.code.oss-x86_64.flatpak
  $ flatpak run com.visualstudio.code.oss

(Removing "--user" would install the application system-wide. Also note that
the "org.freedesktop.Platform" runtime needs to be installeed, check the
Flatpak website for instructions.)

The "flatpak-bundler" development dependency provides an asynchronous API to
invoke "flatpak-builder". The "gulp-image-resize" module is used to
incorporate scaling of the application icon to multiple sizes, and uses
ImageMagick (or GraphicsMagick) under the hood, which is commonly available on
most GNU/Linux distributions.

Instead of building the Electron dependencies ourselves, this uses the
io.atom.electron.BaseApp bundle as base application.

Instead of building the Electron dependencies, this reuses the
"io.atom.electron.BaseApp" bundle as base application. The contents of the
base application are imported at build time, and then Electron and the
prepared files added on top. The prebuilt base application will be fetched
automatically using the URL specified in the "baseFlatpakref" attribute of the
manifest, and it is kindly hosted at Amazon S3 by Endless Computers
(https://endlessm.com). The sources for the base application can be found at:

  https://github.com/endlessm/electron-flatpak-base-app/

Fixes #17133

Fixes #15913

Fixes #11542

* electron 1.4.6

* fix broken DND image feedback

* OSS input

* remove cat.exe as it is no longer needed

* update electron.d.ts

* set new ELECTRON_NO_ASAR environment for forked processes

* Automatically enable high contrast theme when enabled in Windows (fixes #15360)

This reverts commit 22d1f42f.

Bash on Ubuntu errors out during launch using the newer version, reverting
so it can be investigated first.

This upgrade includes an upgrade of winpty from a 0.2.x to 0.3.1

This bumps the vscode-textmate version to pick up this work: https://github.com/Microsoft/vscode-textmate/pull/30

Taking in this change allows us to take in fixes such as #14626, and also addresses some exisitng edge cases

#10587

Bumps vscode-textmate to 2.3 in order to pick up this work: https://github.com/Microsoft/vscode-textmate/pull/26

Closes #14294