centralize gdb command result class checking

each command can have only one of two legitimate responses: "error" or -
depending on the command, and thus declared via a flag - "done" or
"running".
this is way nicer than sprinkling the code with else-ifs (where elses
are sufficient) and asserts all over the place - and silently failing in
release builds.

src/plugins/debugger/gdb/gdbengine.h

@@ -176,7 +176,8 @@ public: // otherwise the Qt flag macros are unhappy
         Discardable = 2,
         RebuildModel = 4,
         WatchUpdate = Discardable | RebuildModel,
-        EmbedToken = 8
+        EmbedToken = 8,
+        RunRequest = 16
     };
     Q_DECLARE_FLAGS(GdbCommandFlags, GdbCommandFlag)