Panel in node editor with most often used nodes

This might improve usability:
Have a panel in the tools section of the node editor (the area on the left) that shows a list of buttons with the node types that are most often added by the user.
This list could be generated in several ways:

• Every time the node tree is edited, check if a node was added, and if yes, add the type to the list of most recently used node types (or place this check in the init method of the LuxCoreNode class)
• Or: Iterate over all nodes in the current tree and check which types are the most common
• And/or: show a few hardcoded node types that are known to be often used, e.g. mix material, imagemap, colormix, math etc.

The reason is that while it is good to have nodes grouped by categories, in practice it's also good to have them grouped by how often they are used.

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Start by examining the node editor's tools section on the left and the LuxCoreNode initialization path mentioned in the issue. Resolve whether usage should be tracked from edits, derived from the current tree, or partly hardcoded; done means the chosen approach presents frequently used node types as buttons in that panel.