Fix several thumbnailing issues

It looks like during thumbnail generation permanent 'size-allocate' events did trigger 'thunar_standard_view_size_allocate' which itself stopped any pending thumbnail loading by using 'thunar_standard_view_schedule_thumbnail_timeout'

Using 'thunar_standard_view_request_thumbnails_real' instead, which just attempts to load thumbnails, fixes the bug

Furthermore, the thumbnailer only sets the passed 'request' if it was already set … that does not make much sense.

Fixes #843 (closed) Fixes #933 (closed)

Hopefully as well fixes: Issue #921 (closed) Issue #223 (closed)