RFR: 8286269: [java.desktop] Use newer instanceof language feature wherever possible

[Kevin Rushforth]

On Sat, 8 Jan 2022 18:13:45 GMT, SWinxy <duke at openjdk.java.net> wrote:

> The old java.desktop package needs dusting off. It hasn't been kept up to pace with the Java language, but it should. The purpose of this PR is to replace most or all `instanceof` checks with the new `instanceof` with a variable. Some methods that used `instanceof` were also reworked alongside the change (such as `.equals()`) if they were small and they could be clearer.
> There should be no actual change in outcomes from this PR. I have triple checked my work. Some cases are kinda 'wut?'.
> This is also a large change. When pattern matching for switch arrives, many methods can be reworked to take advantage of it (when it comes.. eventually ;w;).

For future contributions, I invite you to familiarize yourself with the [OpenJDK Contributing Guide](https://openjdk.java.net/guide/), and particularly the ["Socialize your change"](https://openjdk.java.net/guide/#socialize-your-change) section, which highlights the importance of discussing non-trivial changes ahead of time on the appropriate mailing list (client-libs-dev "at" openjdk.java.net, in this case), rather than just showing up with a pull request.

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PR: https://git.openjdk.java.net/jdk/pull/7000