rawinput: custom threshold for BAT (button-analog type) (#654)

## Link to GitHub Issue or related Pull Request, if one exists
#653 

## Description of change
By default, BAT Positive / Negative binds trigger when the stick is
tilted 10% off the center.

This needs a bit more customization since:

1. User may want to customize the center point (e.g., guitar controller
neutral position)
2. User may want to adjust the sensitivity (e.g., guitar controller tilt
angle to trigger wailing).

## Testing
WIP

src/spice2x/cfg/button.h

@@ -56,6 +56,7 @@ private:
     ButtonAnalogType analog_type = BAT_NONE;
     double debounce_up = 0.0;
     double debounce_down = 0.0;
+    int bat_threshold = 0; // for positive/negative only, value in percentage, zero is default
     bool invert = false;
     bool is_temporary = false;
 
@@ -109,6 +110,7 @@ public:
         alternatives.clear();
         device_identifier = "";
         analog_type = BAT_NONE;
+        bat_threshold = 0;
     }
 
     std::string getDisplayString(rawinput::RawInputManager* manager);
@@ -169,6 +171,14 @@ public:
         this->debounce_down = debounce_time_down;
     }
 
+    inline void setBatThreshold(int bat_threshold) {
+        this->bat_threshold = bat_threshold;
+    }
+
+    inline int getBatThreshold() const {
+        return this->bat_threshold;
+    }
+
     inline bool getInvert() const {
         return this->invert;
     }