Class decompiler doesn't use the contributed decompiler

[fbricon]

Following on #2985, when opening a random .class, the internal disassembler is always used, ignoring the configured decompiler.

Steps To Reproduce

1. Download, unzip and opened this inconsistent-decompiler.zip
2. Make sure the "dgileadi.java-decompiler" extension is installed.
3. Open src/foo/Bar.java
4. Ctrl+Click on BouncyCastleProvider, the source was generated by the Fernflower decompiler
5. Open bin/XMLServerLauncher.class

Current Result

The XMLServerLauncher source is generated by the JDT disassembler, instead of fernflower, so only contains stub methods

Expected Result

The XMLServerLauncher source should be generated by the configured "java.contentProvider.preferred" decompiler, fernflower in this case.