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

[Harshitha Onkar]

FrameBorder and DialogBorder had border scaling issues similar to JInternalFrame. This fix addresses it by creating `AbstractMetalBorder` class 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.

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Commit messages:
 - 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

Changes: https://git.openjdk.org/jdk/pull/12391/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12391&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294484
  Stats: 786 lines in 3 files changed: 389 ins; 327 del; 70 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