TaskTree - employ TaskTree for new usages

Feel free to add any possible new TaskTree usages to the list below:

1. Android plugin - as a continuation after sending AndroidSdkDownloader::sdkExtracted() signal. It looks like continuations are series of asynchronous tasks, like:
   • AndroidSdkManagerPrivate::reloadSdkPackages() - this executes a blocking process with sdkManagerCommand() currently.
   • The latter emits AndroidSdkManager::packageReloadFinished(), and it calls AndroidSettingsWidget::downloadOpenSslRepo(), which starts a git clone process asynchronously.
2. RunControl
   • could the Runworker system completely replaced by tasktree?
3. Debugger plugin
   • could individual command/response handling be tasks? there are "mini state machines" for e.g. setting and updated
   • setups using debug servers have typically two processes running in parallel
   • "mixed" debugging has two DebuggerEngines running in parallel
4. Make a VcsCommand revolution:
   • Most probably remove VcsCommand and use Utils::Process directly.
   • Instead of VcsCommand::addJob() functionality, use TaskTree
   • Get rid of Process::setExitCodeInterpreter() and tweak the process return value in the TaskTree instead
   • Introduce much more TaskTree usages, as there are many cases that some processes are invoked in a blocking way from the main thread and later the asynchronous one is used (after receiving the data from the previous one)