general: Add placeholder text to various search entries

HIG says that all text entries must have a placeholder text or a label, so this change adds a placeholder text to all search entries from Settings.

https://developer.gnome.org/hig/patterns/controls/text-fields.html#guidelines

Revival of !1526 (closed). The majority of the strings were suggested by Allan in !1526 (comment 1609984).

Fixes #2190 (closed)