PWG stuffs: allow displaying recent images outside the recent period

The "Recent pictures" was previously taking only pictures from the recent
period. For example, when no pictures were recently added, the block was
empty.

This patch adds a "Only display images from the recent period"
configuration checkbox (ticked by default to preserve the original
behavior). Unchecking it allows taking "the N most recent pictures",
regardless of the recent period. This way, the block will never be empty
(except if the gallery is actually empty).
!empty($_POST['recent_period_only']) is simpler and is less browser-dependent (this is the way we test checkboxes in Piwigo config for example)