KeyValuePage: configurable items per page

Tweak building to start from items per page instead of
a fixed item height.
Guess the best font size that fit.
Update separator specification from using a "----" to
the now generic separator=true (this allows not wasting
a slot for each separator in the page and not have
only 12 items and 2 small lines in a 14 items page).

frontend/apps/reader/modules/readerwikipedia.lua

@@ -106,6 +106,7 @@ function ReaderWikipedia:addToMainMenu(menu_items)
             end
             UIManager:show(KeyValuePage:new{
                 title = _("Wikipedia history"),
+                value_overflow_align = "right",
                 kv_pairs = kv_pairs,
             })
         end,