[calamares]: Improve startup logging

- Don't bother with the address of the main thread
 - Do put a marker on restart into the log file
 - Do put the Calamares version into the log file (previously, the version
   was printed through cDebug() before the log file was opened, so it was
   lost to the on-disk log).

src/calamares/CalamaresApplication.cpp

@@ -65,9 +65,6 @@ CalamaresApplication::CalamaresApplication( int& argc, char* argv[] )
 void
 CalamaresApplication::init()
 {
-    cDebug() << "CalamaresApplication thread:" << thread();
-
-    //TODO: Icon loader
     Logger::setupLogfile();
 
     setQuitOnLastWindowClosed( false );

src/libcalamares/utils/Logger.cpp

@@ -31,6 +31,7 @@
 #include <QVariant>
 
 #include "utils/CalamaresUtils.h"
+#include "CalamaresVersion.h"
 
 #define LOGFILE_SIZE 1024 * 256
 
@@ -145,9 +146,18 @@ setupLogfile()
         }
     }
 
+    // Since the log isn't open yet, this probably only goes to stdout
     cDebug() << "Using log file:" << logFile();
 
+    // Lock while (re-)opening the logfile
+    {
+    QMutexLocker lock( &s_mutex );
     logfile.open( logFile().toLocal8Bit(), std::ios::app );
+    if ( logfile.tellp() )
+        logfile << "\n\n" << std::endl;
+    logfile << "=== START CALAMARES " << CALAMARES_VERSION << std::endl;
+    }
+
     qInstallMessageHandler( CalamaresLogHandler );
 }