Add a class for keyboard input

Removes the windows-specific hack from sceCtrl
to read keyboard input and instead add a hack to
PPSSPPWindows.

The input mapping is still hardcoded and is the same
as it was in sceCtrl.

Add an std::list to WindowsHost that, curretly, contains
a hardcoded list of preferred `InputDevice`s. Devices
are tried in-order until one of them reports success.
Xinput is preferred over Keyboard. Keyboard always
returns success. Pointers are stored in the std::list
for polymorphism.

Change XinputDevice to report failure as soon as a
device can't be queried, instead of only a frame later.

Windows/KeyboardDevice.cpp

@@ -0,0 +1,46 @@
+#include "KeyboardDevice.h"
+#include "../Common/CommonTypes.h"
+#include "../Core/HLE/sceCtrl.h"
+#include "WinUser.h"
+
+static const unsigned short key_ctrl_map[] = {
+	VK_SPACE, CTRL_START,
+	'V',      CTRL_SELECT,
+	'A',      CTRL_SQUARE,
+	'S',      CTRL_TRIANGLE,
+	'X',      CTRL_CIRCLE,
+	'Z',      CTRL_CROSS,
+	'Q',      CTRL_LTRIGGER,
+	'W',      CTRL_RTRIGGER,
+	VK_UP,    CTRL_UP,
+	VK_DOWN,  CTRL_DOWN,
+	VK_LEFT,  CTRL_LEFT,
+	VK_RIGHT, CTRL_RIGHT,
+};
+int KeyboardDevice::UpdateState() {
+	float analogX = 0;
+	float analogY = 0;
+	for (int i = 0; i < sizeof(key_ctrl_map)/sizeof(key_ctrl_map[0]); i += 2) {
+		if (!GetAsyncKeyState(key_ctrl_map[i]))
+			__CtrlButtonUp(key_ctrl_map[i+1]);
+		else {
+			__CtrlButtonDown(key_ctrl_map[i+1]);
+			switch (key_ctrl_map[i]) {
+			case VK_UP:
+				analogY -= .8f;
+				break;
+			case VK_DOWN:
+				analogY += .8f;
+				break;
+			case VK_LEFT:
+				analogX -= .8f;
+				break;
+			case VK_RIGHT:
+				analogX += .8f;
+				break;
+			}
+		}
+	}
+	__CtrlSetAnalog(analogX, analogY);
+	return 0;
+}