Fixes #17162

Support building Flatpak application bundles

Use license in product.json for rpm

#17073

#17073

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 #16639

fixes #16709

Allow to use middle mouse click in trees (fixes #14453)

fixes #16939

fixes #16999

Fixes #17133

Improve rpm dependencies

Fixes #17142

Add GConf2 dependency in rpm spec template

Fixes #17138