Open file paths in focused window
Track AtomWindow from ipc open event

This ensures the window where the file is dropped is used to open
that file into even when that window does not have focus.
:lipstick:
:memo: Doc dev mode case
Use once for closed event
:memo: Move comment above isFile check
:memo: Doc file/folder path behavior in usage message
Open folder in current untitled window

Untitled windows will now have their project path set when a folder
is opened.
:memo: Doc other atom.open options
:memo: Clarify safeMode description
Rename topWindow to lastFocusedWindow
Merge pull request #3355 from atom/ks-open-files-in-current-window

Open file paths in focused window