Winforms: Replace "magic" strings in Cursors.cs with const strings declared at top of file

[zsd4yr]

actual:

```c#
public static Cursor HSplit {
get {
if (hSplit == null)
hSplit = new Cursor("hsplit.cur", 0);
return hSplit;
}
}

expected:

```c#
internal const string HSPLIT_CUR = "hsplit.cur" 

// ...

public static Cursor HSplit {
            get {
                if (hSplit == null)
                    hSplit      = new Cursor(HSPLIT_CUR , 0);
                return hSplit;
            }
        }

[ericstj]

HSPLIT_CUR doesn't exactly follow our coding guidelines for constants https://github.com/dotnet/corefx/blob/master/Documentation/coding-guidelines/coding-style.md

Moreover you may want to close this in preference to #140.

[willibrandon]

Hi, first time contributor here and I'm currently learning how to use GitHub. I've committed the requested changes to my repository and just saw ericstj's comment regarding coding guidelines and possibly closing this issue. How should I proceed?

[karelz]

@willibrandon let's wait on clarification if this change is needed. @zsd4yr @Tanya-Solyanik can you please help clarify if we should close this issue in favor of #140 as suggested above?

[zsd4yr]

We’re moving in favor of #140 because it will allow us to take advantage of the high resolution and accessibility complaint cursors in the OS. @willibrandon I’m glad you’re helping out here :) I expect there will be additional entry-level issues soon

Closing this issue in favor of #140

[Tanya-Solyanik]

Hi @karelz. I discussed this with @zsd4yr , we hit a race condition opening these 2 issues. Indeed #140 is the preferred way to implement this - we will get all OS improvements(high res and the right system theme) when we access the system cursors :) and will reduce size of our binary somewhat.

[karelz]

Thanks!