Expose API for custom tooltips/dropdowns & cell value setters/getters

Hi,

one more; a nice feature would be expose functionality that allows the consumer to display custom tooltips and dropdowns when editing cells (including plain text, no formulas); and ideally a way to read and modify cell content in spite of a cell being edited / Excel being in busy state.

Potential use cases could be anything related to autocompletion, including intellisense features for code typed into regular Excel cells;