Mail codebase needs a cleanup to fix several issues

[stippi]

• Mail is not font sensitive and overwrites the user configured system fontsize for some (but not all) controls
• the code is quite messy and ignores proper object oriented programming concepts
• the print settings message is used from deleted print jobs instead of being copied
• the code is riddled with race conditions when it comes to changing settings:
  • program settings are implemented via global variables, in different status of having been cleaned up
  • program settings are applied at "random" with locking problems and race conditions
  • some program settings are available to change in the preference GUI, but have no effect, since the respective code is not in use anymore (for example disabling the text under the icon buttons)

[alexbl]

Related to #4495

[waddlesplash]

Most of the mail system now uses the Layout API. Close?

[stippi]

Using the Layout API was only one bullet point. I don't think the others have been addressed. Please leave open.