Issue244: GetDefaultEntry returns first expression, without semicolon

From #244

When GUI commands choose which expression to use (Plot2d..., Simplify...) etc, and a cell is active but no selection is made, GetDefaultEntry would copy the entire text of the active cell, but this always wrong because it includes the semicolon or dollar sign, which causes a syntax error.

This patch fixes this issue, as well as only allowing the first expression in a cell to be copied in, because these functions cannot deal with multiple expressions either.