Make view edges style-able

[hlfcoding]

This is a modest request to fill in some theming gaps to make LimeChat even more customize-able.

Note that in the current UI the view edges (some of which are drag handles) are thick and always a light gray. My request is to provide support (probably though yml) to be able to change the color and width of those edges. This is especially useful for putting content first on white-on-black themes, especially given the scrollbar UI changes since Lion.

[GabLeRoux]

👍

[ronilaukkarinen]

Big up for this.