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

actual:

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

expected:

internal const string HSPLIT_CUR = "hsplit.cur" 

// ...

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

[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!