Add mouse input option for the third-person camera (#1011)

* Untie existing mouse logic from gyro

* A bit more mouse cleanup before I start building off it

* Rebase and last bit of cleanup

* Fix rebase mistake, don't apply invertFirstPerson to gyro or mouse input

* Remove the deprecated ImGui toast system

* Add Mouse Camera option in preparation for its use

* WIP, add mouse controls for the third-person camera

* Various helpText revisions

* Enable free camera on horseback

* Untie mouse camera and free camera options
Either being enabled now allows the underlying freecam logic to run

* Allow simultaneous C-stick and mouse input

* Combine mouse sensitivities for both aim and camera

* Add option for inverting mouse Y

* Refactor cursor visibility handling

* Tighten aim capture condition and constrain cursor to window region

* Tidying my trash

* Last bit of housekeeping so I'm satisfied

* Don't write code while sleep deprived

* Fix my sloppy merge and a few helpText updates

* Disable control stick aim when mouse aim is active

* Use same conditions for cursor grabbing as for capture

include/d/actor/d_a_alink.h

@@ -4551,7 +4551,7 @@ public:
 #if TARGET_PC
     void handleWolfHowl();
     void handleQuickTransform();
-    bool checkGyroAimContext();
+    bool checkAimContext();
 
     void onIronBallChainInterpCallback();
 

include/d/d_camera.h

@@ -1037,6 +1037,7 @@ public:
     bool test1Camera(s32);
     bool test2Camera(s32);
     #if TARGET_PC
+    static bool canUseFreeCam();
     bool freeCamera();
     bool executeDebugFlyCam();
     void deactivateDebugFlyCam();