Allow an icon rule to put multiple icons on a column
Fill with transparent instead of white, and fix silly typo.
Merge branch 'kovidgoyal/master'
Make icon rules composable.
Get rid of over-elaborate cache naming
Allow multiple icons for column icons

Column icons: Allow the use of multiple icons with column icon rules.
You can now have column icon rules display multiple icons in a single
column, side by side. There are two ways to do this, either specify
multiple icons when creating the rule, or create multiple rules that
match the same book and specify the icon type to be 'composed' for every
rule. See Preferences->Look & Feel->Column icons for details.

Merge branch 'master' of https://github.com/cbhaley/calibre