Fix swiftlint build errors and a lot of warnings

[NSMutableString]

Still 80 to go... but coming closer to 0. ;-)

[NSMutableString]

58 to go...

[NSMutableString]

54 to go.

[vincentneo]

@NSMutableString,
It's my first time using SwiftLint. I have reduced a little more of the issues.
I wanted to doing my part of clearing some of the mess that I did along the way 😅.

Please do correct me if I have done something wrong, by the way.

[vincentneo]

@merlos,
Now that we are down to 12 warnings, should merge or continue?

For some warnings I kinda did not want to solve the warning for the sake solving (like function body length violation by a few lines more than recommended)

For some, I don't know how to resolve.

[merlos]

Thank you guys. I had never used SwiftLint and it helps for having a better code.

PS: Sorry, it took me a while to review.