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/document/documentregistry.lua

@@ -72,10 +72,7 @@ function DocumentRegistry:hasProvider(file, mimetype)
     end
     local DocSettings = require("docsettings")
     if DocSettings:hasSidecarFile(file) then
-        local doc_settings_provider = DocSettings:open(file):readSetting("provider")
-        if doc_settings_provider then
-            return true
-        end
+        return DocSettings:open(file):has("provider")
     end
     return false
 end
@@ -134,7 +131,8 @@ function DocumentRegistry:getProviders(file)
         local added = false
         local suffix = string.sub(file, -string.len(provider.extension) - 1)
         if string.lower(suffix) == "."..provider.extension then
-            for i, prov_prev in ipairs(providers) do
+            for i = #providers, 1, -1 do
+                local prov_prev = providers[i]
                 if prov_prev.provider == provider.provider then
                     if prov_prev.weight >= provider.weight then
                         added = true
@@ -143,7 +141,7 @@ function DocumentRegistry:getProviders(file)
                     end
                 end
             end
-        -- if extension == provider.extension then
+            -- if extension == provider.extension then
             -- stick highest weighted provider at the front
             if not added and #providers >= 1 and provider.weight > providers[1].weight then
                 table.insert(providers, 1, provider)