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/games/io.h

@@ -5,6 +5,16 @@
 
 namespace games {
 
+    namespace ModifierButtons {
+        enum {
+            Modifier1,
+            Modifier2,
+            Modifier3,
+            Modifier4,
+            Size,
+        };
+    }
+
     namespace OverlayButtons {
         enum {
             Screenshot,
@@ -59,6 +69,7 @@ namespace games {
 
     const std::vector<std::string> &get_games();
     std::vector<Button> *get_buttons(const std::string &game);
+    std::vector<Button> *get_buttons_modifiers(const std::string &game);
     std::string get_buttons_help(const std::string &game);
     std::string get_analogs_help(const std::string &game);
     std::vector<Button> *get_buttons_keypads(const std::string &game);