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/InputDevice.cpp

@@ -0,0 +1,13 @@
+#include "InputDevice.h"
+#include "XinputDevice.h"
+#include "KeyboardDevice.h"
+#include <list>
+#include <memory>
+
+#define PUSH_BACK(Cls) do { list.push_back(std::shared_ptr<InputDevice>(new Cls())); } while (0)
+std::list<std::shared_ptr<InputDevice>> getInputDevices() {
+	std::list<std::shared_ptr<InputDevice>> list;
+	PUSH_BACK(XinputDevice);
+	PUSH_BACK(KeyboardDevice);
+	return list;
+}