deletion functionality for mass edition

fix #1349 and #1241 by adding deletion functionality to mass edition. It adds checkboxes for each field in the mass edit layout, which allows delete fields by just checking the checkbox without modifying the field contents.

![mass_edition_delete](https://code.vtiger.com/vtiger/vtigercrm/uploads/8209714be9f273bd405218414c5cdc1e/mass_edition_delete.gif)

See merge request !679