Model.ts refactor

[ghost]

Description of the Pull Request

Major refactoring of the model.ts file. Multiple signals in the widget.ts file replaced with a single more general signal. Name changed from CSVTabularDataEditor to Tabular Data Editor.

Issue being fixed:

None.

Changes proposed:

1. In model.ts created an interface ICoordinates which defines an object with a row and a column
   In model.ts, created method sliceOut which removes the entry at a given Coordinate.
2. In model.ts created method insertAt which adds a value to a given Coordinate.
3. In model.ts created methods to assist the work of sliceOut and insertAt
4. In model.ts modified setData, addRow,... to utilize sliceOut and insertAt.
5. In model.ts removed setField as it's function can be handled by applications of sliceOut and then insertAt.
6. In widget.ts, bundled signals _addRow, _addColumn,... into a single signal _changeModel
7. In index.ts changed contextMenu commands to use _changeModel.

[ghost]

Something I forgot to mention in the description: We can avoid the hassle of shifting when calling getOffsetIndex entirely by seeking indices towards then end first. So for example, if we are removing a column, rather than having to do
for(row=1; row <=model.rowCount('body'); row++) { //some annoying shifting stuff
we just do
for(row=model.rowCount('body'); row>0; row--) { // nice clean code

[thegooman]

Iterating backward to avoid needing to shift is a genius idea. Good thought!