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.cpp

@@ -299,7 +299,27 @@ std::string Button::getMidiNoteString() {
     return fmt::format("{}{}", note_names[index % 12], ((index / 12) - 1));
 }
 
-std::string Button::getDisplayString(rawinput::RawInputManager* manager) {
+std::string Button::getDisplayString(rawinput::RawInputManager *manager) {
+    auto binding_display = this->getBindingDisplayString(manager);
+    auto modifiers = this->getModifierMask();
+    if (binding_display.empty() || modifiers == 0) {
+        return binding_display;
+    }
+
+    std::string modifier_display;
+    for (unsigned int index = 1; modifiers != 0; index++, modifiers >>= 1) {
+        if ((modifiers & UINT8_C(1)) == 0) {
+            continue;
+        }
+        if (!modifier_display.empty()) {
+            modifier_display += "+";
+        }
+        modifier_display += fmt::format("Mod{}", index);
+    }
+    return modifier_display + "+" + binding_display;
+}
+
+std::string Button::getBindingDisplayString(rawinput::RawInputManager *manager) {
 
     // get VKey string
     auto vKey = (uint16_t) this->getVKey();