app: merge app/internal/wm into package app

The app and app/internal/wm packages are tightly coupled, requiring
quite a bit of forwarding types, values and constants from the internal
package to export it. Further, no other package imports package wm.

This change merges the two packages.

While here, drop the pre-Go 1.14 SIGPIPE workaround.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/framework_ios.h

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+#include <UIKit/UIKit.h>
+
+@interface GioViewController : UIViewController
+@end