Add function to run code in page scope

This is necessary, because since firefox 33 access to the page's js scope
is very limited and does not allow for complex modifications.
We can bypass this by executing the code that makes the changes itself in the
page scope. This is done by appending a script tag with the serialized
code to the page dom.
Refactor video resolution enforcement to support ff33+; remove fexp code

The unsafeWindow API got restricted in firefox 33 and so some operations didn't
work anymore. Basically it is no longer allowed to expose objects from the
privileged addon scope to the page scope. All objects have to be cloned using
the structured clone algorithm, which does not support meta-programming like
properties. (similar to serializing to json)
This commit introdues an approach that works around this limitation by using
the function introduced in the last commit to run serialized functions in the
page scope, which then in turn are able to construct complex objects in the page
scope.

All static values, simple objects and functions are exported using the new
mechanisms (createObjectIn, cloneInto, exportFunction) provided since firefox 33,
only the code that constructs the youtube config injector is run in the page
scope using the function described above.

As a side effect, this commit removes the code that removed the youtube
experiment which was responsible for fixed video sizes, because it is now
implemented permanently and cannot be disabled anymore.
Another solution for the fixed video sizes will be introduced in the next commit.
Partially fix the problem with fixed size videos

Youtube permanently switched to using fixed sized videos. This results in the
video being shown too small when using the option to increase the size of the
video player.
This commit introduces some css that pertially fixes the problem: The video
itself is now displayed properly, however the progress bar and annotations
are not. It is based on an idea by Yonezpt in the issue linked below.

There is currently no simple solution to this problem, because all possible
solutions either break other functionality or involve injecting code into
the minified youtube player, which is very hard to maintain. (and ugly)

See here for more details: https://github.com/YePpHa/YouTubeCenter/issues/1083
Rename yt-player-size to yt-player-height and update setting labels

Size and height were used inconsistently in this setting, now we cleanly
separate between user view and code.
Fix title when starting videos paused and improve pausing in general