LuaSettings: Add a method to initialize a setting properly (#7371)

* LuaSettings/DocSettings: Updated readSetting API to allow proper initialization to default.
Use it to initialize tables, e.g., fixing corner-cases in readerFooter that could prevent settings from being saved.
(Fixes an issue reported on Gitter).
* LuaSettings/DocSettings: Add simpler API than the the flip* ones to toggle boolean settings.
* Update LuaSettings/DocSettigns usage throughout the codebase to use the dedicated boolean methods wher appropriate, and clean up some of the more mind-bending uses.
* FileChooser: Implement an extended default exclusion list (fix #2360)
* ScreenSaver: Refactor to avoid the pile of kludges this was threatening to become. Code should be easier to follow and use, and fallbacks now behave as expected (fix #4418).

frontend/ui/data/settings_migration.lua

@@ -25,8 +25,8 @@ function SettingsMigration:migrateSettings(config)
     end
 
     -- Fine-grained CRe margins (#4945)
-    local old_margins = config:readSetting("copt_page_margins")
-    if old_margins then
+    if config:has("copt_page_margins") then
+        local old_margins = config:readSetting("copt_page_margins")
         logger.info("Migrating old", cfg_class, "CRe margin settings: L", old_margins[1], "T", old_margins[2], "R", old_margins[3], "B", old_margins[4])
         -- Format was: {left, top, right, bottom}
         config:saveSetting("copt_h_page_margins", {old_margins[1], old_margins[3]})
@@ -39,7 +39,7 @@ function SettingsMigration:migrateSettings(config)
     -- Space condensing to Word spacing
     -- From a single number (space condensing) to a table of 2 numbers ({space width scale, space condensing}).
     -- Be conservative and don't change space width scale: use 100%
-    if not config:readSetting("copt_word_spacing") and config:readSetting("copt_space_condensing") then
+    if config:hasNot("copt_word_spacing") and config:has("copt_space_condensing") then
         local space_condensing = config:readSetting("copt_space_condensing")
         logger.info("Migrating old", cfg_class, "CRe space condensing:", space_condensing)
         config:saveSetting("copt_word_spacing", { 100, space_condensing })