added mixer for standard quad vtol plane in SITL gazebo

ROMFS/px4fmu_common/mixers/standard_vtol_sitl.main.mix

+Mixer for standard vtol plane (SITL) with motor x configuration
+=========================================================
+
+This file defines a single mixer for a standard vtol plane (SITL gazebo) with motors in X configuration.
+The plane has two ailerons and one elevator. The ailerons and elevator are treated as elevons
+in order to make the standard vtol simulation compatible with the tailsitter simulation.
+
+R: 4x 10000 10000 10000 0
+
+# mixer for the elevons
+M: 2
+O:      10000  10000   0 -10000  10000
+S: 1 0  5000   5000    0 -10000  10000
+S: 1 1 -5000 -5000     0 -10000  10000
+
+M: 2
+O:       10000   10000   0 -10000  10000
+S: 1 0   5000    5000    0 -10000  10000
+S: 1 1   5000    5000    0 -10000  10000
+
+# mixer for the pusher/puller throttle
+M: 1
+O:      10000  10000      0 -10000  10000
+S: 1 3      0  20000 -10000 -10000  10000