New actions: view next|prev

[MaxGyver83]

Add actions to view the next tag(s) to the right or the left side. Bind them to C-PageUp/PageDown, by default.

Examples:

• If tag 1 is selected, view next shows all windows on tag 2 (same as view 2).
• If tag 2 is selected, view next shows all windows on tag 3 (same as view 3).
• If the last tag is selected, view next shows all windows on tag 1 (same as view 1).
• If tags 2 and 4 are selected, view next shows all windows on tags 3 and 5 (same as view 3 + viewtoggle 5).
• Analogous for view prev: If tag 2 is selected, view prev shows all windows on tag 1 (same as view 1).

[rpmohn]

This sounds potentially useful, thanks! I'll think some more about it. Sorry I've been silent, busy with a couple of other things. I'll get to this!