New horizontalLayout function; put elements in a single table row
Use standardized widths for selectize/jslider

The 100% width worked well inside of a sidebar, but in other situations
like full-width columns or zero-min-width tables a fixed width is better.
If there's demand we can add parameters for setting the width to custom
values including 100%.
Update NEWS
Replace horizontalLayout with flowLayout; add splitLayout
Add inputPanel
Update NEWS, tweak description for flowLayout
Tweak padding
Merge pull request #465 from rstudio/feature/horizontal-layout

New layouts for horizontal placement