ddr: add support for analog axis-only pads and handle left+right / up+down input (#659)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Some crappy DDR soft mats (either USB-based or console pads with crappy
adapters that lack a dedicated "DDR pad mode") use X and Y analog axis
for arrows, which means they cannot cleanly report Up+Down or Left+Right
input.

According to some ancient Stepmania code, a lot of these report a middle
value (value between center and right, for example, for left+right).
Borrow this hack from Stepmania and implement it in spice as analog
input for DDR.

Also, for rawinput/xinput automatic bind scenario, update the logic so
that face buttons are always detected before analog axis / dpads for
consistency. This would help with binding any DDR pads that
simultaneously input face buttons / dpad / analog axis, so that we can
prefer face buttons when detected.

## Testing
I don't have any soft mats that have this, but it was tested with an
xbox controller.

src/spice2x/games/ddr/io.h

@@ -33,6 +33,16 @@ namespace games::ddr {
         };
     }
 
+    // all analogs in correct order
+    namespace Analogs {
+        enum {
+            P1_LEFT_RIGHT,
+            P1_UP_DOWN,
+            P2_LEFT_RIGHT,
+            P2_UP_DOWN
+        };
+    }
+
     // all lights in correct order
     namespace Lights {
         enum {
@@ -177,5 +187,8 @@ namespace games::ddr {
 
     // getters
     std::vector<Button> &get_buttons();
+    std::string get_buttons_help();
+    std::string get_analogs_help();
+    std::vector<Analog> &get_analogs();
     std::vector<Light> &get_lights();
 }