Add 360p and 1440p as possible player sizes (+refactoring)

The setting is now called player size instead of video size, which was confusing
because there is another setting that really sets the video size (quality).
Properly implement volume fix and pause of start option

This is done by using a callback function that is called by the youtube code when
the player is ready and the API can be used.
Define available player sizes in main script

This way we can use these in several content scripts.
Properly force video resolution by changing the player config

This works by changing the player config before the player starts (inspired by
YePpHa's YouTubeCenter (https://github.com/YePpHa/YouTubeCenter)
Additionally the setting is now called "YouTube video resolution" and not
"YouTube video quality", which causes it to be reset to the default.
Fix player size issues (progress bar) by disabling experiment 931983

This experiment programmatically sets the size of several player ui elements like
the progress bar, which does not work when resizing the player window or requires
crude workarounds like the scaling of the annotations.
However it is very possible that youtube enables this experiment by default one
day so that we would have to use the workarounds again.
Thanks to YePpHa who found this: YePpHa/YouTubeCenter@fcffe2601f426bc4344deb522ba1d4b8e6e1e8d3
Make button smaller and add tooltip that explains the action
Update readme with newest contributers and fix spelling
Bump version to 2.2.0beta2