Renamed "apercu" to "minipreview" in the source code
To be consistent, all place where the not-properly-written "apercu" was
used in the source code (of wmaker and WPrefs) it has been replaced by an
appropriate "minipreview" or similar, to be in line with the new name
suggested by Yuri Tarasievich.
This new name is better understood by contributors who speak usual english,
but not this word which comes From french but is sparsely understood by
british people.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>