Ability to add ToolStrips to the main UI

Adding elements to the Editor's ToolStrip is simple, however there is only so much horizontal space to place things.
I think it would be useful if it were possible to add new ToolStrips, which would appear below the default one.
Additionally, it would make sense for separate EditorPlugins to be able to have their own dedicated ToolStrips. Otherwise there is little guarantee that their elements would fit on the screen, considering that there could be any number of other added elements vying for the same room.

To distinguish user-added ToolStrips from the Editor's, I would suggest using the term "UserToolStrip", or similar, in the api
```cs
//Adding a new ToolStrip
var myToolStrip = Editor.Instance.UI.AddUserToolStrip();
//Getting all added ToolStrips
var myToolStrips = Editor.Instance.UI.UserToolStrips;
```