enable style merge behavior between parent-child components (fix #3997)

[defcc]

fix #3997

Style priority orders as bellow:

1. binding style > static style.
2. parent style > child style.

[defcc]

Something wrong with background-image parsing. I'll update it later.

[defcc]

Will use a regexp to parse if background property included.

background:url(http://xxx.jpg);
background-image:url(data:image/webp;base64,UklGRkoAAABXRUJQVlA4WAoAAAAQAAAAAAAAAAAAQUxQSAwAAAARBxAR/Q9ERP8DAABWUDggGAAAABQBAJ0BKgEAAQAAAP4AAA3AAP7mtQAAAA==)

[yyx990803]

Thanks! There are a few places where I think there could be some perf improvement, but it's a bit hard to explain so I'll merge and tweak.

[defcc]

Thanks @yyx990803 ，I'll take a look.