jupyter: Add virtual framebuffer X server dependency (#1827)

* Add xvfb (virtual framebuffer X server) as a dependency to Binder apt file be able to run m.nviz.image from
  notebooks on Binder. This requires xvfb-run or a Xvfb command to run.
* Add PyVirtualDisplay as a Python interface to Xvfb.
* Sort entries in the apt and requirements files.

binder/apt.txt

@@ -4,6 +4,8 @@ bison
 flex
 g++
 gettext
+git
+imagemagick
 libblas-dev
 libbz2-dev
 libcairo2-dev
@@ -27,10 +29,9 @@ libxmu-dev
 libzstd-dev
 make
 netcdf-bin
+p7zip
 proj-bin
 sqlite3
 unixodbc-dev
+xvfb
 zlib1g-dev
-p7zip
-imagemagick
-git

binder/requirements.txt

@@ -1,5 +1,6 @@
-numpy
+folium
 matplotlib
+numpy
 Pillow
 ply
-folium
+PyVirtualDisplay