Restore full filemanager functionality

Prior to introduction of the reader-chooser infrastructure we had a very
useful feature of browsing (and therefore
copying/moving/deleting/renaming/creating) filesystem tree accessing
arbitrary files if the user privilege mode is set to any value higher
than BEGINNER.
This patch restores this functionality by checking the privilege levels
in readerchooser and returning CREReader for any file type if there is
no other reader associated with it (including files with no extension).

readerchooser.lua

@@ -72,12 +72,17 @@ function ReaderChooser:getReaderByType(ftype)
 	if #readers >= 1 then
 		return registry[readers[1]][1]
 	else
-		return nil
+		if FileChooser.filemanager_expert_mode  > FileChooser.BEGINNERS_MODE then
+			return CREReader
+		else
+			return nil
+		end
 	end
 end
 
 function ReaderChooser:getReaderByName(filename)
-	local file_type = string.lower(string.match(filename, ".+%.([^.]+)"))
+	local ext = string.match(filename, ".+%.([^.]+)")
+	local file_type = ext and ext:lower() or "txt"
 	local readers = GetRegisteredReaders(file_type)
 	if #readers > 1 then -- more than one reader are registered with this file type
 		local file_settings = DocSettings:open(filename)
@@ -121,7 +126,11 @@ function ReaderChooser:getReaderByName(filename)
 	elseif #readers == 1 then
 		return registry[readers[1]][1]
 	else
-		return nil
+		if FileChooser.filemanager_expert_mode  > FileChooser.BEGINNERS_MODE then
+			return CREReader
+		else
+			return nil
+		end
 	end
 end