Freak, this is extremely nice code.
Suppose that you normally split your code

Don't know if I would see through 120KB of source code after some time passed by...
I code MDI apps for quite awhile and there are 2 little things I would like to suggest you add into your code ( you don't have to... ) :
1.) Every MDI-Child Window has his own SysMenu.
You can see that at the end there is a: NEXT Ctrl+F6.
This should be functional - you would have to add some piece of code for it.
2.) Another thing is the WINDOW Menu.
Normally if you have a MDI Application you GRAY this MenuTitle when there are no MDI-Child Windows, and ENABLE it if there is at least one.
( the same goes for the Toolbar Buttons 'Tile Vertically/Horizontally and Cascade Windows )
PureBasic does not have a function for disabling the MenuTItle, but it's doable.
Tell me if you need some piece of code for my suggestions
Another thing would be to make the Main Window active after the User closes the Library Designer Preferences Window.
Like I said above - these are only suggestions that you don't have to follow.
Again good work, keep it up man
BTW did I tell you I like your coding style...