[BUGFIX] fix site set settings definitions for view settings

[bahammes]

The current view settings in the Main site set settings definition file are incorrectly defined under plugin.tx_powermail.settings.view.*, which causes them to be ignored.
This patch corrects the configuration keys to the proper format (by removing settings.) so they are recognized and used:

plugin.tx_powermail.settings.view.templateRootPath => plugin.tx_powermail.view.templateRootPath
plugin.tx_powermail.settings.view.partialRootPath => plugin.tx_powermail.view.partialRootPath
plugin.tx_powermail.settings.view.layoutRootPath => plugin.tx_powermail.view.layoutRootPath