ProjectExplorer: Start merging Runnable and StandardRunnable

With AndroidRunnable gone, there is only one incarnation of
a Runnable left - the StandardRunnable.

It is not expected to ever need a different runnable again,
as platform differences are now handled in the platform specific
RunConfigurations and RunWorkers created there locally, and
global information is typically communicated via
RunConfigurationAspects, so there is no point in keeping the
Runnable::{Concept,Model} machinery.

This patch here essentially makes StandardRunnable a type alias
for Runnable, to allow downstream changing all

  if (r.is<StandardRunnable>()) { ... r.as<StandardRunnable>(); }

one by one.

When all downstream is adjusted, the alias can go completely.

Change-Id: I86aa92c7fae8d54ca603484b7e1746c126b0bddb
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/plugins/projectexplorer/devicesupport/sshdeviceprocess.cpp

@@ -280,7 +280,7 @@ void SshDeviceProcess::handleKillOperationTimeout()
     emit finished();
 }
 
-QString SshDeviceProcess::fullCommandLine(const StandardRunnable &runnable) const
+QString SshDeviceProcess::fullCommandLine(const Runnable &runnable) const
 {
     QString cmdLine = runnable.executable;
     if (!runnable.commandLineArguments.isEmpty())