Add a "Add External Tileset" tool button to Tilesets panel #2882

I had a .tsx file that I wanted to use in a map, but none of the tool buttons I was looking at were what I was after:

<img width="156" alt="Screen Shot 2020-08-09 at 10 54 29 am" src="https://user-images.githubusercontent.com/6002340/89728641-0e82e980-da2f-11ea-84ba-d7e376883a0d.png">

Then I found this:

https://discourse.mapeditor.org/t/importing-exporting-tsx/827/2

I think it would be more discoverable and intuitive to also have this option available with the other tool buttons in the Tilesets panel.