Add global parameter filemanager_expert_mode

In order to switch between safe and advanced way for filechooser to manage files, I've introduced special parameter 'filemanager_expert_mode':
i) 'safe' means that filemanager shows only files with the reader-related extentions; make safe renaming (i.e. the extention remains the same)
ii) 'advanced': all files are available (no filtering); any renaming is allowed;
TODO: to add also 'expert' mode & use it for unstable or dangerous functions

reader.lua

@@ -127,6 +127,8 @@ if fontmap ~= nil then
 	Font.fontmap = fontmap
 end
 
+-- set up the mode to manage files
+FileChooser.filemanager_expert_mode = G_reader_settings:readSetting("filemanager_expert_mode") or 1
 -- initialize global settings shared among all readers
 UniReader:initGlobalSettings(G_reader_settings)
 -- initialize specific readers