GHA: Add workflow_dispatch to enable manually triggering builds

[hugovk]

Changes proposed in this pull request:

• This will add a button to allow us to manually trigger a new build for a (whole) workflow, similar to Travis CI
• https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows#manual-events

[nulano]

Good idea. I would suggest changing the condition in test-windows.yml for building wheels from if: "github.event_name == 'push'" to if: "github.event_name != 'pull_request'" as well as the related conditions throughout the workflow - this would enable anyone to trigger a Windows build and download the wheels for any specific revision in their fork without the GPL imagequant dependency - that seems to me more useful than manually triggering a build with imagequant enabled but not generating wheels.

[hugovk]

Updated, how does 915d014 look?

[nulano]

Looks good: https://github.com/nulano/Pillow/actions/runs/1533419665