Adds a do not show again checkbox to the empty install dir warning popup on lib start. This only appears when the install dir is empty to prevent mission scripting replacement.
Also adds a warning message to the log when the dir is empty. Will help to identify this in bug reports.
This will allow expert users to disable the automatic MissionScripting.lua replacement. There are many warnings and errors which have to be ignored to achieve this because DCS Liberation will not work with unmodified MissionScripting.lua
