HIG: Add more widgets to selection controls (#1851)
HIG: Add more widgets to selection controls (#1851)

* HIG: Add more widgets to selection controls

* Rename the Checkboxes & Switches section to "Selection controls" as in the Material HIG
* Add Radio buttons, mode buttons, and comboboxes
* Edit for clarity

* Don't duplicate linked buttons

* Fix fucky IDs

* Don't forget to terminate those spans

* resolve Cassidy's comments

elementary/website,elementary/mvp,elementary/mvp,elementary/website,elementary/website,elementary/mvp