Problem is that USBDeviceAddToWhitelistDialog starts a timer once created to poll for devices every second. In Qt, closing a heap-allocated dialog doesn't delete it, so it keeps on polling. This fix is to allocate dialog on the stack, then use "exec" to run it modally without returning. Once closed, the stack instance will get destroyed, thus killing the timer.
* Removed the Cancel button since the code doesn't react to it anyway.
* Only show a window title, not the help icon (?), and disable the close button
* Set the title to "Dolphin" instead of repeating the label text