Add "Enable GPU hardware acceleration" option

[yuya-oc]

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Summary
Add "Enable GPU hardware acceleration" option.

It's enabled by default.

Issue link
#713

Test Cases

• config.json (covered by CI)

  • enableHardwareAcceleration key is saved when clicking the option in the settings page.

• Actual process

  1. Set Enable GPU hardware acceleration to disabled.
  2. Restart the app.
  3. Check running mattermost-desktop processes.
  4. Processes which have an argument --type=gpu-process should not exist.

Additional Notes
https://circleci.com/gh/yuya-oc/desktop/696#artifacts

[svelle]

Tested own build and CI-Build on Win 10 Pro x64 with AMD 18.3.1 Drivers and no problems so far, toggled GPU acceleration on and off a few times.
Will test Linux (Fedora 27) tomorrow morning and report back.

[jasonblais]

Great! Thanks @yuya-oc

Tested on Mac, one of the three Mattermost processes is removed after disabling the acceleration. I'll use the app with acceleration disabled today to see if there are any issues, but ready to merge from my side.

@amyblais please help test on Windows as well?

[amyblais]

Tested on Windows as well and saw no issues either.

[jasonblais]

I think this one is good to merge then @yuya-oc. Hopefully it helps with the blank page issues

[yuya-oc]

In my Ubuntu 16.04 VM, such process did not originally exist. Possibly because it's VM and hardware acceleration doesn't work well. However it's effective on other OS, so I think it's good to merge.