Xmind-to-Excel exporter: use the specific Excel cell formats instead of "default"

Currently the Xmind-to-Excel exporter uses only default Excel cell format for all data (incl. dates and durations).

I propose that Xmind uses the specific Excel formats as "date" for dates and "time" for durations (with unit "d"). Otherwise the data cannot be directly used for further (Excel) processing.