Make theme_bw truly black and white

And improve some alignment and sizes (ticks width matches panel border width etc.)
Add theme_white

Which is intended to give an aspect similar to theme_grey but with a white background.
Do not overwrite theme_bw and change names
Fix coding style for new themes

Spaces around `=`
Link the documentation of all themes

Using the `@family` keyword of roxygen
Add examples to showcase themes
Add NEWS bit about new themes
Merge pull request #923 from jiho/fix/theme_bw

New themes that do not overwrite existing ones