Fix TabControl's Scroll Bug && Beautify GridView

1. Fix TabControl's scroll bug, when use GetChildAtPoint, always return the first tabpage. Should use GetChildAtPoint with GetChildAtPointSkip.
2. Change GridView's ColumnHeadersBorderStyle in winform, more beautiful.
Hm, I'm not sure if this should be default, as the border at the bottom of the header is a standard look for most apps with a grid..

However, you can customize this for your specific app like so:
Eto.Style.Add<Eto.WinForms.Forms.Controls.GridViewHandler>(null, h => h.Control.ColumnHeadersBorderStyle = swf.DataGridViewHeaderBorderStyle.None);