Plots|DataCombined - load observed data and show data set names

[PavelBal]

Add a button Load data. A window is shown where the user can select an excel file with observed data (default value is the file defined in the project configuration), an importer configuration (default value specified in the project configuration), and check-box list of sheets in the observed data file.

Clicking on "OK" ("load", whatever) does this

https://github.com/esqLABS/esqlabsR/blob/cb7973ddba4160702044e23dfe4dec2f86dfe66e/R/utilities-data.R#L268

Then, in the Data set column, the names of the loaded data sets (returned is a named list so just take the names of the list) are shown as a drop-down.