pygmt.set_display: Support setting image dpi and width globally

### Description of the desired feature

The idea is to let `set_display()` have two more parameters `dpi` and `width` which can change the image dpi and width for all images displayed by the `Figure.show()` calls.

_Some comments originally posted by @weiji14 in https://github.com/GenericMappingTools/pygmt/pull/529#discussion_r594838525_

> Personally I don't see a use-case for setting a global `dpi` or `width` (but happy to be proven wrong). My suggestion is that we only let this `set_display` function set the method and not do anything about the dpi or width. Haven't looked too closely, but I think most of the changes were made in ca0ecb3d649bbf00b380e8848b3c82b9cd28a68c and could be partially reverted there. Not doing anything about the dpi/width would also greatly reduce the diff on this PR and make it easier to review. Discuss first though.

### Are you willing to help implement and maintain this feature?

Maybe