mouseDown now uses cell, column, and row selection modes

[kgoo124]

I am currently working on the Tabular Data Editor extension with @lmcnichols and @ryuntalan. We noticed the BasicMouseHandler doesn't utilize the different selection modes onMouseDown. This change would set the proper selectionMode during the onMouseDown() function. There are other places in the mouse handler where the grid is selecting certain areas, so those could also be changed as well.

Fixes #113

[blink1073]

Thanks!

[blink1073]

cc @nmichaud I released the following changes:

 - @lumino/application: 1.10.4 => 1.11.0
 - @lumino/datagrid: 0.12.0 => 0.13.0
 - @lumino/datastore: 0.10.3 => 0.11.0
 - @lumino/default-theme: 0.4.4 => 0.5.0
 - @lumino/widgets: 1.13.4 => 1.14.0