Add OCC command to reset rendered texts

The command is meant to be used when the fonts used to render texts
("core/fonts/NotoSans-Regular.ttf" and "core/fonts/NotoSans-Bold.ttf")
are changed (for example, to add support for other scripts). The avatar
and text file previews will be removed, so they will be generated again
with the updated font when needed.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>