Generalize file list locators so a specific ancestor can be used

The file list is used in other places besides the Files app (for
example, the File sharing app); in those cases the locators for the file
list elements are the same, but not for the ancestor of the file list.
To make possible to reuse the file list locators in those cases too now
they receive the ancestor to use.

Note that the locators for the file actions menu were not using an
ancestor locator because it is expected that there is only one file
actions menu at a time in the whole page; that may change in the future,
but for the time being it is a valid assumption and thus the ancestor
was not added to those locators in this commit.

Although the locators were generalized the steps themselves still use
the "FilesAppContext::currentSectionMainView" locator as ancestor; the
steps will be generalized in a following commit.

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