Add ability to swap viewport

Right now view consists of viewports and this list is somewhat fixed.

One could add a tag to `vPortCreate()` so that newly created viewport mimics given viewport's dimensions etc. and is not added to view's viewport list. But there's a problem with viewport managers - when they are working on copblocks, they create them and they are added to copperlist's copblock list. So by initializing viewport manager on detached viewport the copblock would automatically be added to processing during every frame. That could be fixed by setting copblocks to disabled state, but still it gets more and more hacky from there.

This is probably not fixable in the near future, but who knows, perhaps there will be other reasons to make yet another view system rewrite. ;)