[Everything] Use multiple instances of Everything

**Is your feature request related to a problem? Please describe.**

Ability to query multiple instances of Everything, i.e. local instance + ETP server. Game changing if you want to search items on external server.
[Everything docs](https://www.voidtools.com/support/everything/multiple_instances/) state that Named instances can be used for that. Here is example command line to start two named instances:
```
"C:\Program Files\Everything\Everything.exe" -instance "Local" && "C:\Program Files\Everything\Everything.exe" -instance "ETPServer"
```

Each instance would pick up it's configuration from `C:\Program Files\Everything\Everything-Local.ini` and `C:\Program Files\Everything\Everything-ETPServer.ini` correspondingly. You may even do: `Everything.exe && Everything.exe -instance "ETPServer` to launch default instance + extra. User is supposed to configure each instance from Everything UI.

**Describe the solution you'd like**

Ability to specify multiple paths with instances in `Plugins -> Explorer -> Everything Settings -> Everything Path`. Maybe a list instead of one textbox, or one textbox with `;` separated values.

In Flow, send search query to all instances at once and combine the results.

**Describe alternatives you've considered**

In settings of Everything itself there is no such options to combine query results from different instances. So we can't use single command line for that. Also in Everything if you connect to ETP server directly in one instance, you won't be able to search local file system in that same instance. So two separate Everything instances are required.

**Additional context**