Back in the days (2.4x and before), it was rather easy to get some
invalid utf-8 strings in Blender. This is totally breaking modern code,
so this commit adds a simple 'check & fix strings' operator, available
from the main File menu.
Not much to add, you can now clear previews from current .blend file, or a set of non-opened files.
Likewise, you can generate previews (for mat/tex, objects, groups, scenes, ...).
