RFR: 8263095: Provide a way for a custom control to indicate that its userAgentStyleSheet has changed

[Alessadro Parisi]

This change allows custom control to change their style dynamically
When the user-agent stylesheet changes the property automatically calls `NodeHelper.reapplyCSS(Region.this);` to recompute the CSS for the node and its children.
To make this work the StyleManager class must be fixed too.
The line `regionUserAgentStylesheet = weakRegionUserAgentStylesheetMap.computeIfAbsent((Region)region, Region::getUserAgentStylesheet);
` is problematic because if the region is already present in the map but the user-agent stylesheet changed, the value of `regionUserAgentStylesheet` is not updated and this leads to controls having the wrong style.
To fix this issue I added this check:

if (((Region) region).getUserAgentStylesheet() != null && !((Region) region).getUserAgentStylesheet().equals(regionUserAgentStylesheet)) {
    weakRegionUserAgentStylesheetMap.put((Region) region, ((Region) region).getUserAgentStylesheet());
    regionUserAgentStylesheet = ((Region) region).getUserAgentStylesheet();
}

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Commit messages:
 - Allow dynamic implementation of user-agent stylesheet

Changes: https://git.openjdk.java.net/jfx/pull/525/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=525&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263095
  Stats: 44 lines in 2 files changed: 24 ins; 17 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/525.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/525/head:pull/525

PR: https://git.openjdk.java.net/jfx/pull/525