Statistics spreadsheet for spreadsheets

sometimes it's useful to have the statistical properties of columns like min, max, etc. in a separate spreadsheet that also be used to plot these values. S.a. the discussion in #31 (closed). This is available for example in Scidavis:

Additionally to our "Column statistics"-Dialog we can offer a similar functionality - via a new entry in the context menu for "Column statistics spreadsheet" we create a new spreadsheet a put all the available statistical values into this table like structure. We can add this new spreadsheet "Statistics" as a child to the source spreadsheet so that the relation between them is clear. In the project explorer it would like in this screenshot: