Implemented GUI with zenity

Now the program is completely functional and is using zenity dialogues.
`sources` directory was renamed to `src`. UI related stuff was moved to
`src/gui/ui`.

src/ui-socket.h

@@ -0,0 +1,22 @@
+
+/*
+ * Interface for controlling communication with the UI.
+ */
+
+#ifndef UI_SOCKET_H
+#define UI_SOCKET_H
+
+#include <sys/types.h>
+
+struct process_info {
+  pid_t PID;
+  const char *name;
+  uid_t UID;
+};
+
+// For default socket location, set socket_path = NULL.
+int init_ui_socket(const char *socket_path);
+
+int ask_access(const char *filename, struct process_info pi);
+
+#endif // !UI_SOCKET_H