Add two keyboard actions for focusing on next/previous view in splt view mode

This commit adds two keyboard action for focusing on next or previous view in split view tab.

Th default shortcuts are ctrl-F11 for next view and ctrl=shift-F11 for previous view. I find these shortcuts more useful in some situations than the currently availble shortcuts for moving the focus up/down/left/right.

The order of the views in the list is the one returned by ViewSplitter::findChildren<TerminalDisplay *>, so it does not correlate exactly to their positions.