Incompatibilities between Eclipse JDT 4.26 and 4.27

So far Eclipse did not change incompatibly between 4.26 and 4.27 in ways that affect plug-ins. Plug-ins that ran on 4.26 should run on 4.27 without any problems.

Note about ECJ separation from JDT Core

ECJ (Eclipse compiler for Java) code is moved from org.eclipse.jdt.core to dedicated org.eclipse.jdt.core.compiler.batch bundle and will be deployed as a separated bundle. The org.eclipse.jdt.core.compiler.batch is now included in SDK as a regular Eclipse bundle and can be compiled / deployed / used separately from org.eclipse.jdt.core bundle. All of ECJ packages are re-exported by org.eclipse.jdt.core, therefore from OSGI point of view, all 3rd party code that used some compiler related API from org.eclipse.jdt.core doesn't require any change. The org.eclipse.jdt.core.compiler.batch bundle itself doesn't have any dependencies and so can be used in Eclipse products that do not use workspace concepts.

However, no change is without side effects.

Known problems with the split of the ECJ from core bundle

As part of the org.eclipse.jdt.core.compiler.batch code separation from org.eclipse.jdt.core, the two fragments of org.eclipse.jdt.core - org.eclipse.jdt.compiler.apt and org.eclipse.jdt.compiler.tool were merged into org.eclipse.jdt.core.compiler.batch. So if some target definition, launch configuration or build file referenced the two fragments, these references can and should be removed now.
Another issue might affect standalone (non OSGI based) applications that were using org.eclipse.jdt.core as a "simple" Java library (which jdt.core never was). So for example code that had org.eclipse.jdt.core_XYZ.jar on classpath and tried to call this outside Eclipse:
```
ASTParser parser = ASTParser.newParser(AST.getJLSLatest());
```
would fail now with
```
NoClassDefFoundError: org.eclipse.jdt.internal.compiler.env.ICompilationUnit
```
because org.eclipse.jdt.core.dom.ASTParser uses internally ECJ APIs and they are now ... surprise ... moved to org.eclipse.jdt.core.compiler.batch jar. To fix this error, org.eclipse.jdt.core.compiler.batch library should be added to the application classpath.