Fixed issue #24

I think this bug is caused by following fact. If `DiagramDialog::slotDeleteGraph()` is called when `Delete` button is pressed, it always clears `GraphInput` line edit contents. Then it emits signal `textChanged` and initiates `slotRestToTake`. This slot clears graph variable, because input field is cleared. But selected item remains in `GraphList`. And it seems that variable is not cleared. 

I propose the next solution. Now `DiagramDialog::slotDeleteGraph()` sets in `GraphInput` the last remaining variable from the `GraphList`. If there is no remaining items, it sets empty string. This way works for me. There is no invalid curves now.