Remove MULTILINE() macro hack

I have a macro for writing [multiline](https://github.com/FireFlyForLife/NanaDesigner/blob/master/NanaDesigner/Utils.h#L4) strings. 

But as time went on C++ now has raw string literals 
```cpp
R"(Even quotes are allowed here!
And I can just keep typing with new
lines!)"
```

Places where I use this:
 - hardcoded divs.
 - export templates (for header/source files)