APM_Control: factor out an AP_PitchRollController base class

[peterbarker]

Saves ~100 bytes on MatekF405

[peterbarker]

Binary Name      Text [B]        Data [B]     BSS (B)        Total Flash Change [B] (%)      Flash Free After PR (B)
---------------  --------------  -----------  -------------  ----------------------------  -------------------------
antennatracker   0 (0.0000%)     0 (0.0000%)  0 (0.0000%)    0 (0.0000%)                                      340584
ardusub          0 (0.0000%)     0 (0.0000%)  0 (0.0000%)    0 (0.0000%)                                      176100
arducopter-heli  0 (0.0000%)     0 (0.0000%)  0 (0.0000%)    0 (0.0000%)                                       46032
blimp            0 (0.0000%)     0 (0.0000%)  0 (0.0000%)    0 (0.0000%)                                      320884
ardurover        0 (0.0000%)     0 (0.0000%)  0 (0.0000%)    0 (0.0000%)                                      140824
arduplane        -92 (-0.0096%)  0 (0.0000%)  -4 (-0.0031%)  -92 (-0.0095%)                                    19760
arducopter       0 (0.0000%)     0 (0.0000%)  0 (0.0000%)    0 (0.0000%)                                       55720

[tridge]

This will need a lot of testing of all the APM_Control functionality