Allow to use Spotify CEF prebuilt binaries when building cefpython

[cztomczak]

Currently there are two custom patches applied to CEF that are required to build cefpython. These patches are to be merged into upstream, pull requests were already created. There is still some work required on these patches. Issue251 patch needs to have a unit test for it to be accepeted. Issue231 patch needs to be reworked again due to issue with CEF unit test runner.

Currently as of v57/v58 cefpython can be built by using ready binaries from Spotify on Windows and Mac. Linux is not yet supported.

Spotify CEF prebuilt binaries can be downloaded from:
http://opensource.spotify.com/cefbuilds/index.html

TODO:

• Issue Discovery of the "icudtl.dat" file fails on Linux. Merge patch into upstream. #231
• Issue Drag & drop in OSR mode (Kivy). Expose API on all platforms. #251
• Update Workflow when updating CEF version #264 NOTES

[cztomczak]

For v55 releases on Windows/Mac Spotify builds are used.