remove superfluous engine mapping

JsCommandExecutorThreadObject is running in exactly one thread now.
There's no need to save engines per thread anymore.

Change-Id: I9328cf840789131397e7713efdc105f0b56aeee8
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
simplify JS command execution

Do not modify the global object.
Use pushScope/popScope instead of pushContext/popContext.
Remove a futile caching attempt.

Change-Id: I2c7440356db13db25c1770e9461ef6247d1bb7dc
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
provide "compile single file" functionality

To compile just one single file without any further effects,
pass the file name to BuildOptions::setChangedFiles, and the
file tags of the desired results to BuildOptions::setActiveFileTags.

Task-number: QBS-283
Change-Id: I70213b248e1e07fd7524a54f7f2e4f250621967f
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>