Reorganize bottom menu config panels (#6131)

Mostly for the PDF bottom menu.
- Reorganize by topic, trying to limit the number
  of widgets per panel to 4.
- Re-order some toggles from low to high ('off' then 'on').
- Show font size as number instead of a list of "Aa".
- PDF: add more font size values, and increase usable
  contrast values.
- Add help_text to most PDF toggle titles.
- CRE line spacing: increase fine tuning min and max.
Also avoid zoom advice messages on book load.

frontend/ui/widget/button.lua

@@ -37,6 +37,7 @@ local Button = InputContainer:new{
     preselect = false,
     callback = nil,
     enabled = true,
+    allow_hold_when_disabled = false,
     margin = 0,
     bordersize = Size.border.button,
     background = Blitbuffer.COLOR_WHITE,
@@ -261,7 +262,7 @@ function Button:onTapSelectButton()
 end
 
 function Button:onHoldSelectButton()
-    if self.enabled and self.hold_callback then
+    if self.hold_callback and (self.enabled or self.allow_hold_when_disabled) then
         self.hold_callback()
     elseif self.hold_input then
         self:onInput(self.hold_input, true)
@@ -277,7 +278,7 @@ function Button:onHoldReleaseSelectButton()
     -- Safe-guard for when used inside a MovableContainer,
     -- which would handle HoldRelease and process it like
     -- a Hold if we wouldn't return true here
-    if self.enabled and self.hold_callback then
+    if self.hold_callback and (self.enabled or self.allow_hold_when_disabled) then
         return true
     elseif self.hold_input or type(self.hold_input_func) == "function" then
         return true