Blender Git Loki

Filebrowser: Expose file select functions for File Browser to Python API.

This patch adds activate_file_by_relative_path(relative_path="") and
deselect_all() function to the space api of the File Browser. While the
first sets the active file and adds it to the selection based on a
relative path to the current File Browser directory the second one
deselects all files but does not change the active file.

Differential Revision: https://developer.blender.org/D12826
Reviewed by: Julian Eisel