Introduce current_name_{set|get}

Introduce current_name_{set|get}

Summary:
When fileselector is opened, it would be nice to be able to provide initial filename.
Like "Untitled Document" in Gedit, when save for the first time.

More details in Gedit,
Gedit provies default name, "Untitled Document" and does not change default name
while navigating directories.
Once we selected any file, Gedit changes value of name entry to selected filename and
it is not also changed while navigating directories.

@feature

Reviewers: seoz, yakov-g

Reviewed By: yakov-g

Subscribers: seoz

Maniphest Tasks: T1664

Differential Revision: https://phab.enlightenment.org/D1504