Use string_view in more places and reduce the number of empty string literals

Evil Eyerequested to merge
Assumeru/openmw:guiew into master

Part of #6125.

Most of this MR is dedicated to easing the transition to the upcoming MyGUI version that adds string_view support to most of its API. (See https://gitlab.com/Assumeru/openmw/-/commits/mystringvui and https://github.com/MyGUI/mygui/pull/252 )

Edited by Evil Eye

Merge request reports