add share button and implement AirDrop share in OSX, update info.plist to set on Mac OS the default app for PPI, on iOS include the Documents/Inbox subfolder used to store the received files from AirDrop

PanoPainter-OSX/main.h

@@ -9,13 +9,14 @@
 @interface Controller : NSWindowController<NSWindowDelegate>
 @end
 
-@interface View : NSOpenGLView
+@interface View : NSOpenGLView<NSSharingServiceDelegate>
 {
     @public Window* wnd;
     CVDisplayLinkRef dl;
     NSOpenGLContext* glctx;
     _CGLContextObject* cgl;
     bool gl_ready;
+    @public NSString* file2open;
 }
 - (void)close;
 - (void)terminateGL;
@@ -24,4 +25,5 @@
 - (void)async_swap;
 - (std::string)pick_file:(NSArray<NSString*>*)types;
 - (std::string)pick_dir;
+- (void)share_file:(NSString*)file_path;
 @end