app: add code for macOS and Windows apps under 'app' (#12933)

* app: add code for macOS and Windows apps under 'app'

* app: add readme

* app: windows and linux only for now

* ci: fix ui CI validation

---------

Co-authored-by: jmorganca <jmorganca@gmail.com>

app/dialog/cocoa/dlg.h

@@ -0,0 +1,43 @@
+#include <objc/NSObjCRuntime.h>
+
+typedef enum {
+	MSG_YESNO,
+	MSG_ERROR,
+	MSG_INFO,
+} AlertStyle;
+
+typedef struct {
+	char* msg;
+	char* title;
+	AlertStyle style;
+} AlertDlgParams;
+
+#define LOADDLG 0
+#define SAVEDLG 1
+#define DIRDLG 2 // browse for directory
+
+typedef struct {
+	int mode; /* which dialog style to invoke (see earlier defines) */
+	char* buf; /* buffer to store selected file */
+	int nbuf; /* number of bytes allocated at buf */
+	char* title; /* title for dialog box (can be nil) */
+	void** exts; /* list of valid extensions (elements actual type is NSString*) */
+	int numext; /* number of items in exts */
+	int relaxext; /* allow other extensions? */
+	char* startDir; /* directory to start in (can be nil) */
+	char* filename; /* default filename for dialog box (can be nil) */
+	int showHidden; /* show hidden files? */
+	int allowMultiple; /* allow multiple file selection? */
+} FileDlgParams;
+
+typedef enum {
+	DLG_OK,
+	DLG_CANCEL,
+	DLG_URLFAIL,
+} DlgResult;
+
+DlgResult alertDlg(AlertDlgParams*);
+DlgResult fileDlg(FileDlgParams*);
+
+void* NSStr(void* buf, int len);
+void NSRelease(void* obj);