[K7][UI Framework]: Pagination rows example, text style for ButtonEmpty

[snide]

This PR adds the following:

• Examples for how to handle a rows per page "dropdown". Examples added both to pagination docs and to context menu.
• Assorted icons, including an "empty" icon. One that is an empty SVG that can take the place of an real icon when you need to line things up.
• A new type for KuiButtonEmpty called "text" which styles a button like normal text (this is good for repetitive links like pagination or navigation).
• Some cleanup of pagination CSS to use the above, more modular components.

TODO in a non-breaking followup PR

• Figure out a method to color the popover "arrow" when titles aren't used.

[snide]

This is a cheat, but I think one we'll likely need over and over again. Sometimes you want to line up a row of items, and only show an icon (say a check) on the active row. While you could toggle classes to achieve the same thing, it's much easier (and scalable) to just have an "empty" icon which takes up the space of an icon, but contains no real content.

[cjcenizal]

FYI I've simplified the KuiContextMenu's interface for cases where you only need a single panel in https://github.com/elastic/kibana/pull/14183/files#diff-f821d1d03e3b86a67081d403bcca5003R66. When that's merged I'll rebase the K7 branch and update the example introduced by this PR.

[cjcenizal]

🎁 LGTM

[chrisronline]

Yay!