LPS-43009 Hook to support RTL feature in 6.2.x (2nd part)

Hey Brian,

This is the second set of commits for the RTL hook. I've considered your comments to my previous pull-request and also added the RTL tool logic we need to support custom theme plugins. Next and last set are mainly bug fix backports. 

Regarding the README, I'll send the final version (including how to use the tool) to Jim in a separate commit.

Thanks!