ExtraCompiler: Expose TaskItem for compile task

This is an intermediate state before employing one common TaskTree
inside CppProjectUpdater. Use multiple one-task TaskTrees for now.

Refactor ExtraCompiler so that there is only one pure virtual method
to be implemented (taskItem()) instead of two (2 run() overloads).
Use TaskTree inside ExtraCompiler for running the compilation process.

Change-Id: I6884934508e043594589d117f6d3f0aed94b84c2
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/python/pythonlanguageclient.cpp

@@ -249,7 +249,7 @@ void PyLSClient::updateExtraCompilers(ProjectExplorer::Project *project,
                         updateExtraCompilerContents(extraCompiler, file);
                     });
             if (extraCompiler->isDirty())
-                static_cast<ExtraCompiler *>(extraCompiler)->run();
+                extraCompiler->compileFile();
         } else {
             m_extraCompilers[project] << oldCompilers.takeAt(index);
         }