RFR: JDK-8294484: MetalBorder's FrameBorder & DialogBorder have border rendering issues when scaled [v4]

[Harshitha Onkar]

> FrameBorder and DialogBorder had border scaling issues similar to JInternalFrame. This fix addresses it by creating `AbstractMetalBorder` class within MetalBorder, that contains the common steps required for painting border for `FrameBorder`, `DialogBorder` and `InternalFrameBorder`. 
> 
> All 3 cases - JFrame, JDialog and JInternalFrame are combined into a single test case - `ScaledMetalBorderTest` and hence the older `InternalFrameBorderTest` which is no longer required, is deleted.

Harshitha Onkar has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 10 additional commits since the last revision:

 - review changes: AbstractMetalWindowBorder, strokeWidth rename
 - Merge branch 'master' into MetalBorderRefactor_8294484
 - minor change
 - removed unused import
 - added updateColor(), isActive() to MetalBorder
 - test summary changes
 - deleted InternalFrameBorderTest
 - moved corner var to AbstractMetalBorder
 - sealed abstract class, record changes
 - initial commit, consolidated test case

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12391/files
  - new: https://git.openjdk.org/jdk/pull/12391/files/ab1e6e79..4be01e84

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12391&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12391&range=02-03

  Stats: 10696 lines in 326 files changed: 4314 ins; 2543 del; 3839 mod
  Patch: https://git.openjdk.org/jdk/pull/12391.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12391/head:pull/12391

PR: https://git.openjdk.org/jdk/pull/12391