Add Flat LAF & IntelliJ themes support capability in JMeter

[KingRabbid]

Description

I've introduced support for Flat LAF Themes in JMeter; Flat LAF is an open source https://www.formdev.com/flatlaf/ Java Swing library with modern look & feel, supports IntelliJ themes, HiDPI, etc (see the website).
If's compatible with JMeter codebase by default.
New parameters are defined & customizable in .properties & .xml file(s): one can adjust the JSyntaxTextArea.java theme defaults.
Few additional bugs/enhancements were also introduced in order to make the visual experience better.

For future: I would remove Darcula and DarkLaf from JMeter, doesn't make sense anymore to keep them; replace DarkLaf features/usage with FlatLAF API where possible.

Motivation and Context

Due to recent eye vision problems, I've started to need alternative themes support to use other colors; existing capability in JMeter was limited.
DarkLaf API currently in-use in JMeter seems to be non maintained (last update September 2022) and complicated to upgrade to latest version.

How Has This Been Tested?

Manually tested with various JMXs & Themes, especially with JSR223 test elements to validate the JSyntaxTextArea instances.
I've run tests on both Linux and Windows, with Java 8, 11 & 17.

Note: changes are fully compatible also with JMeter 5.x branch/version if needed.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.

[KingRabbid]

Applied latest changes, any feedback please?

[vlsi]

DarkLaf API currently in-use in JMeter seems to be non maintained (last update September 2022) and complicated to upgrade to latest version.

Darklaf is not very active, however, the upgrade to the latest version was not really hard: #5715

Having multiple LaFs turned out to be hard to support. They have different quirks, and I agree if we switch to Flat LAF, we'd better drop Darklaf.

@KingRabbid , Could you elaborate a bit on Flat LAF vs Darklaf in your experience?

[KingRabbid]

Hi,

there's little to detail on versus:
a. FlatLAF is actively maintained and used in some major projects so with a very good chance to continue.
b. DarkLAF provides a small subset of LAFs from what is available in the FlatLAF.
c. FlatLAF allows one to define own theme and tweak via .xml settings a theme definition.
d. FlatLAF has custom libraries to work on major OS names in a more compatible manner than a generic Java Swing API.

This branch/repot contains also some few tweaks/fixes that make RSyntaxArea more compatible to LAF changes.