RFR: 8272870: Add convenience factory methods for border and background [v3]

[Nir Lisker]

On Thu, 9 Sep 2021 23:43:29 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

>> Added convenience factory factory methods for Background and Border.
>
> Nir Lisker has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Removed whitespaces
>  - Added tests and doc updates

Added a couple of simple tests.

My approach to the documentation here is that these are the simplest ways to create a border (background) where you provide only 1 color/gradient and not care about the rest, and what you are going to get is rather obvious. If you want to see what the implementation does and what all the defaults are, the `@implSpec` section has a link that you can follow.
I prefer not to start giving these details since they end up taking more attention than those few important details of what the method does. When I tried to expand upon the behavior of the method it quickly became "noise", with details I didn't care for.

If people insist I will add them, but think about: is what you get not what you expected to get with the current docs? What surprised you or what were you not sure about?

-------------

PR: https://git.openjdk.java.net/jfx/pull/610