WPrefs: moved the list of icon animations into an array

By grouping the information together, it makes the code less prone to
errors and easier to maintain as it links things that are related.

Took opportunity to log a message for the case where an invalid value
was found to ease user's debug.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
WPrefs: created an array to store the database values for the Icon Position setting

By having an array, it makes the code simpler in many places, thus easier
to maintain. Took opportunity to include a comment about the trick used to
have a value convenient for use.

It is the opportunity to log an error message to user instead of silently
accepting invalid values.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
WPrefs: grouped the choices for Window Resize Display in a single place

By having an array, it makes the code simpler in many places, thus easier
to maintain and to implement new possibilities.

It is the opportunity to log an error message to user instead of silently
accepting invalid values.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
WPrefs: grouped the choices for Window Move Display in a single place

By having an array, it makes the code simpler in many places, thus easier
to maintain and to implement new possibilities.

It is the opportunity to log an error message to user instead of silently
accepting invalid values.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
WPrefs: add an image to represent the window in the Window Placement frame

The original square box did not look like anything, by using an image that
looks like a small window it is more clear to users what it represents.

The image was drawn as an XPM keeping a style consistent with everything
else; it was then converted to TIFF using ImageMagick:
  convert -depth 8 -compress lzw xpm/smallwindow.xpm tiff/smallwindow.tiff

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
WPrefs: add possibility to configure the size of the aperçu

The Icon preference panel have been rearranged to include a slider which
controls the size of the Aperçu. This slider is also used to turn off the
feature, so the related checkbox have been removed from the Misc preference
panel, because it is more convenient to have the related settings at the
same place.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
WPrefs: Clarify purpose of "window snapping" feature in expert panel.
Translations: Dutch .po files updated

Recently added/changed msgid's were translated, plus a few cosmetic
improvements. Kept one entry for last stable release.
WPrefs: make a text string translatable

The WPrefs miniwindow shows a shorter title that could be translated.