feat: style manager for separate elements

[Rast1234]

Hi. As discussed in #7 i found a way to manage styles for separate windows (any element, in general). Basically it's same StyleManager but without auto-update event and it takes an element to manage.

Example app included.

I am not sure if you are interested in this so i did not write any docs. Feel free to edit, rename, clean up codestyle, add docs, etc if you want to merge.

[Aldaviva]

Thanks, I'll try to take a look at this. I appreciate you sharing it.