Auto detect the API from the trace.

Saves key-presses on the gui for EGL traces.

Might be handy to implement a top level "apitrace retrace" command too.
diff --git a/common/trace_parser.hpp b/common/trace_parser.hpp
index d8c5915..43c1356 100644
--- a/common/trace_parser.hpp
+++ b/common/trace_parser.hpp
@@ -33,6 +33,7 @@
 #include "trace_file.hpp"
 #include "trace_format.hpp"
 #include "trace_model.hpp"
+#include "trace_api.hpp"
 
 
 namespace trace {
@@ -94,6 +95,7 @@
 
 public:
     unsigned long long version;
+    API api;
 
     Parser();