Refactoring of the grid code for the IgxTreeGridComponent

[DDavidkov]

#2530 .

Refactored the IgxGridComponent so the code can be reused by the IgxTreeGrid. Changes include:

• Moving the common code in a grid-common folder
• Adding an interface for the common functionality between the IgxGridComponent and the IgxTreeGridComponent
• Adding a base class for the Grid and the TreeGrid, which contains the common code
• Adding a IgxGridCommonModule
• Moving most of the functionality to the API service, which now is generic and works with the common interface, instead of having a concrete IgxGridComponent dependency