@dc1248's fixes from #18 for pie menu appearing non-3D screens

[tmilker]

This commit is @dc1248's fixes for the issue with the Z pie menu appearing on screens other than the 3D view and not allowing the z key to be pressed in the Script editor. I changed the variable name to be slightly more accurate than _is_active. I also chose it to be true by default since typically Godot opens with the 3D view. This has the side effect of it always working the first time so if you disable/enable the plugin in the Script editor, it will still appear. However, setting it to false means that it never works the first time when you're in the 3D view, which feels worse since the former situation is a lot less likely.

I tried fixing #19, where pressing the Z button again causes the pie menu to disappear but the options are still present and invisible until you press the Escape key. I don't know enough about Godot. I think if the item_cancelled signal could be sent to the pie menu too, that would fix it.

[tmilker]

Well, this still blocks the letter z input in the Inspectors and other areas, so I need to figure out how to make it more specific to just the 3D view I guess.