TFMXImageSlider drag single page

I guess no need to enable dragging when we have only one image loaded.
I suggest to change
```
procedure TFMXImageSlider.MouseDown(Button: TMouseButton; Shift: TShiftState; X, Y: Single);
..
  if (PageCount > 0) and (Button = TMouseButton.mbLeft) then
```
to
`  if (PageCount > 1) and (Button = TMouseButton.mbLeft) then`

