Add a Mutex to the manual software to avoid multiple instances being opened

[PeterBuddAvo]

Currently it is possible to open multiple instances of the manual. This is a particular problem on consoles with hardware help buttons where you can keep opening up new manuals and not closing old ones. It should probably only be possible to open one instance at a time.

[simnedonadni]

Take a look at this "activating the window of primary instance when a second instance starts": https://www.electronjs.org/docs/latest/api/app#apprequestsingleinstancelockadditionaldata