User feedback when ConEmu.xml is note writable

[losandes]

When a user doesn't have permission to write to the ConEmu.xml config file, Save Settings is never disabled. The actual meaning of this is not implicit, and to a new user, among others, it is not clear that the settings weren't saved onchange. Instead, I suggest that you enable the Save Settings button, and provide user feedback in the form of an error message, so they know that they need to gain permission to the file to be able to save their settings. I presented a simple approach to troubleshooting and resolving this in #97.