A few graphics fixes after #4541 (#4554)

* Various FocusManager related tweaks to limit its usage to devices with a DPad, and prevent initial button highlights in Dialogs on devices where it makes no sense (i.e., those without a DPad. And even on DPad devices, I'm not even sure how we'd go about making one of those pop up anyway, because no Touch ;)!).
* One mysterious fix to text-only Buttons so that the flash_ui highlight always works, and always honors `FrameContainer`'s pill shape. (Before that, an unhighlight on a text button with a callback that didn't repaint anything [say, the find first/find last buttons in the Reader's search bar when you're already on the first/last match] would do a square black highlight, and a white pill-shaped unhighlight (leaving the black corners visible)).
The workaround makes *absolutely* no sense to me (as `self[1] -> self.frame`, AFAICT), but it works, and ensures all highlights/unhighlights are pill-shaped, so at least we're not doing maths for rounded corners for nothing ;).

frontend/ui/widget/scrollhtmlwidget.lua

@@ -74,7 +74,7 @@ function ScrollHtmlWidget:init()
         }
     end
 
-    if Device:hasKeyboard() or Device:hasKeys() then
+    if Device:hasKeys() then
         self.key_events = {
             ScrollDown = {{Input.group.PgFwd}, doc = "scroll down"},
             ScrollUp = {{Input.group.PgBack}, doc = "scroll up"},