Fix re-opening file opens empty buffer
Fix re-opening file opens empty buffer

The Tabs container keeps track of all opened filenames to detect
when opening multiple views into the same file.

However, when closing all views into a file, the filename was not
removed, so the backend still thinks there are existing views.

By removing the filename when all views are closed, files are now
re-opened properly.

google/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,google/xi-editor,google/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,modelorganism/xi-editor,google/xi-editor,modelorganism/xi-editor,modelorganism/xi-editor