Set _cellEditorIsModalWindow in qx.ui.table.pane.Scroller

TL;DR: qx.ui.table.pane.Scroller uses a variable named _cellEditorIsModalWindow, but never actually sets it. This PR fixes it.

Long version: I had a need to extend the Scroller class so that I could make tables stop editing fields whenever the cell gets the "focusout" event. Right now, table cells only stop editing when you click elsewhere in the table or if the __cellEditor voluntarily stops the editing (like pressing the enter button). This is a feature that I could consider bringing upstream into qooxdoo, but for now I am fine with using inheritance to extend qooxdoo's existing functionality.

However, since __cellEditor is a private, I can't tell what the cellEditor item is, or whether or not it is a modal window. My logic breaks when using modal window cell editors, as they cause focus to behave much differently.

 I then discovered that there is a variable in the Scroller.js code called _cellEditorIsModalWindow, which from the variable name and the if statement wrapped around it (https://github.com/AaronOpfer/qooxdoo/blob/master/framework/source/class/qx/ui/table/pane/Scroller.js#L1971-L1977), would presumably be a boolean value that is true if the cell editor is a window, and false if otherwise. However, this value is never set, and as a result, there is a codepath that is never taken.

This pull request fixes that so that the value correctly reflects whether the cellEditor is a modal window or not. In addition, it will allow my subclass of Scroller to correctly determine which behavior to use.