Support for Multiple Home Assistant Instances

[june128]

I use multiple Home Assistant instances regularly - I have one at home, one at my parents place, where I need to help out once in a while and one at my local hackspace. Therefore it would be nice, if the app supported multiple instances - just like the iOS (and probably Android and macOS) app does.
This might also give more purpose to the header bar, if one would implement the multi-instance support using tabs in there (see #10).

[cassidyjames]

Great suggestion! It shouldn't be too hard to implement; off the top of my head, I'll want to:

• Change the server setting from a single string to an array of strings
• Change the underlying current page setting to an array of strings
• Redesign the Set Server URL dialog to include multiple dynamic rows
• Add tabs to the header bar if there's more than one server saved
• Add an underlying setting to track which tab is active (can probably just be an int?)