Flatten and simplify Servo's preferences code. In addition, have both
preferences and options passed in as arguments to `Servo::new()` and
make sure not to use the globally set preferences in `servoshell` (as
much as possible now).
Instead of a complex procedural macro to generate preferences, just
expose a very simple derive macro that adds string based getters and
setters.
- All command-line parsing is moved to servoshell.
- There is no longer the concept of a missing preference.
- Preferences no longer have to be part of the resources bundle because
they now have reasonable default values.
- servoshell specific preferences are no longer part of the preferences
exposed by the Servo API.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
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