#122 support for date classes in editor

[m.eik]

i'm fumbling around with some data from a libre office spreadsheet with date columns. R can work with it and perform logical operations without problems.

but the data editor of RKWard just shows the class as "unknown", as it has no support for "Date".

it should be possible to add "Date" to the supported classes and use the "Format" row to define valid format strings, e.g. "%F" to see "2014-03-16" instead of "16135".