Should fix several more bugs.

Realized that program was difficult to read in some terminals.
IE: IDEA IDE terminal is white, and program read fine.
    Linux terminal is dark/black, and was near impossible to read.

Ship order and restock parts now work 100% fine. Issues were entirely due to inconsistent/unexpected database values.

Also fixed related bug with sorted selection queries.

Saving commit to test class-based models

I feel like it's probably better practice to have a few class-level vars that get reused often,
rather than a bunch of local variables that are created at random points as needed.

Thus, variables have been reworked to reflect this. Variables that make sense to have a lower
scope (such as model-specific variables) have been left as-is.

Selection queries now have two methods, depending on if sorted or not.
Also updated commenting for most of DatabaseTransaction class methods.

Now prevents adding discontinued products to orders.
Now appropriately updates UnitsOnOrder values for each added/removed product.
Now allows adding of multiple products to a single order.

Just needs to update "UnitsOnOrder" value for associated product(s).

Validation ended up far messier and more complicated than initially expected.
Now has its own class to improve general code readability.

AddCustomer Menu Item should now be fully functional.

Due to being extra, it is currently unfinished and will remain so until the rest of the program is further fleshed out.

DatabaseConnection class is now handles as a singleton.

Not quite 100% working. Still needs further work/testing.

Appears to be connecting, but need actual database queries to be certain.

Still needs "suitable driver". Trying to figure out what that means.