RFR: JDK-8328750: [TestBug] Improve Stub Font Support

[Marius Hanl]

In https://github.com/openjdk/jfx/pull/1405, I identified some shortcomings of the stub font implementation. As I don't want to clutter the PR with that, I decided to cherrypick the improvements I did to a new ticket and PR.

The current implementation has the following shortcomings:
- It does not reliable detect the System Font, as a consequence, tests in TableColumnHeaderTest.java are failing on my local machine
- Another consequence of this is, that the font size is always estimated with 0, as it is not detected
- One shortcoming currently is, that the stub font siie estimate is not considering bold fonts. That would improve writing tests for some scenarios, e.g. for TableColumnHeader, where we would expect that the size of the header is bigger since it is bold

Some tests were failing for the following reasons:
- `AreaChartTest.java` - `expected -30.0, was -30.00000000004` - I added ceiling to the data.
- `StackedBarChartTest.java` - since we now calculate correctly, the path changed
- A test tried to load `Helvetica`, which is not supported in the stub font loader. I changed it
- The default System font is considered a `Regular` one (style)

I wrote tests and documented the stub behaviour.
I did some minor changes here:
- System font is now detected, also in bold and italic
- A bold font will be calculated with a little bit more width (1px). Checkout the test as well for that

Note: This only changes test setup, no 'production' code.

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Commit messages:
 - improve (stub) font support

Changes: https://git.openjdk.org/jfx/pull/1422/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1422&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328750
  Stats: 316 lines in 8 files changed: 203 ins; 14 del; 99 mod
  Patch: https://git.openjdk.org/jfx/pull/1422.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1422/head:pull/1422

PR: https://git.openjdk.org/jfx/pull/1422