Adds node editor MiniMap feature

[briancairl]

Adds mini map feature to be used between BeginNodeEditor and EndNodeEditor with following features:

• provides "low detail" view of nodes/links in the graph as an overlay in the editor canvas
• provides the ability to zoom in and out of this context to view more of the graph than is visible in the normal editor canvas viewport
• scrolling, which scales with zoom (when zoomed out, scrolls the editor context at a faster rate)
• node snapping when clicked: pans clicked node to center of canvas when mini-map node is clicked
• node hovering callback, which can be used to set custom tooltips when hovering the mini map
• separate color variables for the mini map bg/outline and nodes/links
• four positioning options within the editor canvas

https://user-images.githubusercontent.com/1093236/114410231-0f40c200-9b60-11eb-9057-44e1ac9bbb66.mp4

[briancairl]

@Nelarius I've made the changes as per your suggestions, along with one additional major change https://github.com/Nelarius/imnodes/pull/99/commits/f2ddfa9d4b1b8670e127240ae7df5741f0777e74 to fix poor interact behavior when both the mini-map and a node/link/pin were under the mouse cursor.