SwapView: make PageDown go to next page, PageUp go to previous page

[wkeese]

In SwapView, PageUp goes to the next child, and PageDown goes to the previous child.

Although this corresponds to what's written in SwapView.md, this is the opposite behavior than I expected. It's also opposite from the aria spec, which states:

• Ctrl+PageUp - When focus is inside of a tab panel, pressing Ctrl+PageUp moves focus to the tab of the previous tab in the tab list and activates that tab. When focus is in the first tab panel in the tab list, pressing Ctrl+PageUp will move focus to the last tab in the tab list and activate that tab.
• Ctrl+PageDown When focus is inside of a tab panel, pressing Ctrl+PageDown moves focus to the tab of the next tab in the tab list and activates that tab. When focus is in the last tab panel in the tab list, pressing Ctrl+PageUpwill move focus to the first tab in the tab list and activate that tab.

On a related note, as noted above, SwapView should presumably listen for Ctrl+PageUp and Ctrl+PageDown, not plain PageUp and PageDown, because plain PageUp and PageDown are for scrolling (assuming that the SwapView's child has enough content to cause a scrollbar).