rawinput: modifiers (key combinations) (#813)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #635

## Description of change
Adds optional **button modifiers**, allowing a binding to require one or
more "modifier" buttons to be held before it activates.

- New "Modifiers" controller page in the overlay config to bind the
Modifier 1–4 source buttons.
- Each binding gains a 4-bit modifier_mask (Modifier 1-4). It is
persisted as an optional modifiers XML attribute on button nodes and
controller-preset entries. The attribute is optional and defaults to 0,
so existing config files and presets load unchanged and the feature is
off unless the user opts in.
- The button Edit properties popup gains a "Modifiers" dropdown to pick
which modifiers a binding requires. Doesn't apply to MIDI though.
- Input evaluation skips a binding whose required modifiers are not
held, falling through to its alternatives; velocity reporting uses the
same gated path.
- Controller-preset templates gain a "Modifiers" group for importing and
exporting.

## Testing

src/spice2x/cfg/button.h

@@ -49,6 +49,7 @@ private:
     std::string name;
     std::string device_identifier = "";
     unsigned short vKey = INVALID_VKEY;
+    uint8_t modifier_mask = 0;
 
     // default bindings are always naive
     unsigned short vKey_default = INVALID_VKEY;
@@ -65,6 +66,7 @@ private:
     unsigned short velocity_threshold = 0;
 
     std::string getMidiNoteString();
+    std::string getBindingDisplayString(rawinput::RawInputManager *manager);
 
 public:
     std::string getVKeyString();
@@ -111,6 +113,7 @@ public:
         device_identifier = "";
         analog_type = BAT_NONE;
         bat_threshold = 0;
+        modifier_mask = 0;
     }
 
     std::string getDisplayString(rawinput::RawInputManager* manager);
@@ -123,6 +126,14 @@ public:
         return this->name;
     }
 
+    inline uint8_t getModifierMask() const {
+        return this->modifier_mask;
+    }
+
+    inline void setModifierMask(uint8_t new_modifier_mask) {
+        this->modifier_mask = new_modifier_mask & UINT8_C(0x0F);
+    }
+
     inline const std::string &getDeviceIdentifier() const {
         return this->device_identifier;
     }