Now it's possible to select the material in context menu and new menu to select material.
The patch and workflow has been tested in greasepencil-object branch.
* New Material selector in Draw mode Context menu:
{F8499259}
* Pressing `U`key in Draw mode display material menu.
{F8503224}
Reviewed By: mendio, pepeland
Differential Revision: https://developer.blender.org/D7554