ALL: Move Clipboard support to OSystem

Commit adds kFeatureClipboardSupport. hasTextInClipboard() and
getTextFromClipboard().

OSystem_SDL has this feature if SDL2 is used.

EditableWidget and StorageWizardDialog use g_system to access clipboard
now.

backends/platform/sdl/sdl.cpp

@@ -64,6 +64,11 @@
 #include <SDL/SDL_net.h>
 #endif
 
+#if SDL_VERSION_ATLEAST(2, 0, 0)
+#include <SDL2/SDL.h>
+#include <SDL2/SDL_clipboard.h>
+#endif
+
 OSystem_SDL::OSystem_SDL()
 	:
 #ifdef USE_OPENGL
@@ -171,6 +176,13 @@ void OSystem_SDL::init() {
 
 }
 
+bool OSystem_SDL::hasFeature(Feature f) {
+#if SDL_VERSION_ATLEAST(2, 0, 0)
+	if (f == kFeatureClipboardSupport) return true;
+#endif
+	return ModularBackend::hasFeature(f);
+}
+
 void OSystem_SDL::initBackend() {
 	// Check if backend has not been initialized
 	assert(!_inited);
@@ -453,6 +465,26 @@ Common::String OSystem_SDL::getSystemLanguage() const {
 #endif // USE_DETECTLANG
 }
 
+bool OSystem_SDL::hasTextInClipboard() {
+#if SDL_VERSION_ATLEAST(2, 0, 0)
+	return SDL_HasClipboardText() == SDL_TRUE;
+#else
+	return false;
+#endif
+}
+
+Common::String OSystem_SDL::getTextFromClipboard() {
+	if (!hasTextInClipboard()) return "";
+
+#if SDL_VERSION_ATLEAST(2, 0, 0)
+	char *text = SDL_GetClipboardText();
+	if (text == nullptr) return "";
+	return text;
+#else
+	return "";
+#endif
+}
+
 uint32 OSystem_SDL::getMillis(bool skipRecord) {
 	uint32 millis = SDL_GetTicks();