add auto-refresh of the tree in the tool window

[nils-a]

Currently the tree only loads when the tool-window is opened after a solution is loaded.
For every other case, the tree must be refreshed manually.

Try to hook into VS events (like shown in https://www.visualstudiogeeks.com/blog/visual%20studio%20extensibility/how-to-monitor-solution-events-in-vs-extension) to detect loading/unloading of (additional) projects.