Give more control over CRe margins (#4945)

Without having to resort to weird custom defaults.

* Split the current margins setting in three:
  * Horizontal margins (because you generally want those two to be balanced).
  * Top margin & Bottom margin (because you may want to tweak those separately to deal with quirky status bar/final line shenanigans).

* Also, add a "Reclaim bar height from bottom margin" toggle to the status bar menu, to optionally make sure the status bar won't eat into the bottom margin.

* Includes a free fix to diacritics popup refresh handling in the keyboard ;).

reader.lua

@@ -195,6 +195,10 @@ if Device:hasColorScreen() and not G_reader_settings:has("color_rendering") then
     })
 end
 
+-- Handle global settings migration
+local SettingsMigration = require("ui/data/settings_migration")
+SettingsMigration:migrateSettings(G_reader_settings)
+
 local exit_code
 
 if ARGV[argidx] and ARGV[argidx] ~= "" then