BookMap & PageBrowser: now usable on Non-Touch devices (#12579)

Have them both usable on non-touch devices.
Also:
FrameContainer: fix focus border handling, and draw inner border after the
content (to prevent it from being overridden by the content bgcolor).

frontend/apps/reader/modules/readerthumbnail.lua

@@ -26,7 +26,7 @@ function ReaderThumbnail:init()
         -- The BookMap and PageBrowser widgets depend too much on gestures,
         -- making them work with not enough keys on Non-Touch would be hard and very limited, so
         -- just don't make them available.
-        -- We will only let BookMap run on useDPadAsActionKeys devices.
+        -- We will only let them run on useDPadAsActionKeys devices.
         return
     end
 
@@ -87,8 +87,6 @@ function ReaderThumbnail:addToMainMenu(menu_items)
             self:onShowBookMap(true)
         end,
     }
-    -- PageBrowser still needs some work before we can let it run on non-touch devices with useDPadAsActionKeys
-    if Device:hasDPad() and Device:useDPadAsActionKeys() then return end
     menu_items.page_browser = {
         text = _("Page browser"),
         callback = function()