Support for hover on SplitPane divider

[dar-dev]

It may be useful to have an hover on the SplitPane divider so the user can immediately spot the divider location.

To manage this feature we added a pair of new properties.

SplitPaneDivider.showHover	boolean	optional; default is false
SplitPaneDivider.hoverColor	Color	the divider color on mouse hover if SplitPaneDivider.showHover is enabled

For backwards compatibility this feature must be explicitly enabled.

Here's a simple demo:

splitpane_hover.mp4

Cheers,

Dar

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[DevCharly]

@dar-dev thanks for this PR 👍

I've reviewed it and made some changes:

• removed SplitPaneDivider.showHover:
  To be consistent with other components, hover is now enabled if SplitPaneDivider.hoverColor is specified. Otherwise it is disabled (the default).
• added SplitPaneDivider.pressedColor:
  Used for pressed/dragging state in continuous layout.
  In non-continuous layout, SplitPaneDivider.draggingColor is used as before.
• Fixed painting of dragging divider in non-continuous layout, which was 1px too small and missed the grip dots.
• Updated styling unit tests.

Please test whether it works for you. Thanks.

[MikOfClassX]

@DevCharly thanks for reviewing the code !

@dar-dev mods work flawlessly. just check them. For me is 👍.
In order to contribute it's important to stick to the FlatLaf philosophy as best as we can.
Thanks @DevCharly for pointing out.

[dar-dev]

Thanks for your review, it works perfectly.

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