Tidy up generate_from_settingtypes.lua a bit.

* Multiline strings
* Table-concat instead of String-concats
* string.rep instead of loop-concat
* string.format %q instead of manual quotation by gsub
* Assert writeable files
* Generate new settings_translation_file

src/settings_translation_file.cpp

@@ -1,6 +1,6 @@
 // This file is automatically generated
 // It conatins a bunch of fake gettext calls, to tell xgettext about the strings in config files
-// To update it, refer to the bottom of builtin/mainmenu/tab_settings.lua
+// To update it, refer to the bottom of builtin/mainmenu/dlg_settings_advanced.lua
 
 fake_function() {
 	gettext("Client");