Extract file list locators and steps to its own class

Besides the extraction some minor adjustments (sorting locators for file
action menu entries to reflect the order of the menu entries in the UI,
moving parametrized locators like "createMenuItemFor" above the locators
that use them and placing "descendantOf" calls always in a new line)
were made too.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>