enable Spring symbols dialog shortcut outside of editor

[martinlippert]

Currently, the CMD+6 shortcut to enable the Spring Symbols overlay only works if an editor is selected. It would be nice if that also worked if an element from the Project / Package explorer was selected. The scope of the overlay could be initialized with the scope closest to the selection: if a project is selected, open Spring Symbols in the project scope (same for source folder or package). If a source file is selected, open in file scope.

(cc @odrotbohm)