Prompt to save modified items when closing a pane via pane:close

Fixes #4094
Merge pull request #4129 from atom/ns-fix-pane-close

Prompt to save modified items when closing a pane via pane:close
Add Workspace::paneForItem
:arrow_up: markdown-preview to fix exception when modifying w/ splits
Attach active pane item view before showing/hiding

Fixes #4112
Include inline commands in command palette
Ignore jQuery and duplicates in CommandRegistry::findCommands

Now that jQuery has been patched to add inline listeners and inline
listeners are reported from findCommands, there’s no reason to include
commands based on $.fn.events. Also, we need to ensure the same command
doesn’t get added to the list twice since it could get added by both
inline and selector-based listeners.
Add atom-pane commands via atom.commands again
Include all inline commands on the bubble path in ::findCommands
Don’t run ‘beforeunload’ listeners through atom.commands
Subscribe to ‘window:open-path’ as a command