- `List<String> getAllJavaSourceVersionsSupportedByCompiler()` - all
Java versions that could be used for Java projects inside Eclipse. The
difference to existing `getAllVersions()` API is that later one knows
almost all Java versions ever released and might be used not only in JDT
core but also in debugger/PDE area.
- `boolean isJavaSourceVersionSupportedByCompiler(String version)` -
differs from existing `isSupportedJavaVersion()` in the same way as
explained above
- `String getFirstJavaSourceVersionSupportedByCompiler()` - similar to
existing `latestSupportedJavaVersion()` and should return minimal
"default" version supported by JDT.

API above will be used in JDT UI, Debug (PR's are following), and (most
likely) PDE.

**Internal** API added in batch compiler `CompilerOptions`:
- `getFirstSupportedJavaVersion()`
- `getFirstSupportedJdkLevel()`

See https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2536