WPrefs: minor improvements to the Focus Preference panel

WPrefs: minor improvements to the Focus Preference panel

It is general practice to not consider the strings displayed in a dialog as
real sentences, so this patch removes the dots at the end (including in the
translation);

Took the opportunity to make a light cleanup of the translations in the
concerned areas, and fix some translated string that were probably not
found anymore to due other changes;

Make small reorganisation of the widgets to reduce the risk of truncated
strings, with keeping the overall clean aspect of the panel;

Took the opportunity to clarify the label related to the colormap to avoid
misunderstanding, and include a balloon text to clarify its reason-to-be
because, as modern display tend to all be TrueColor, it may not be
intuitive to new user what it's for.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>