Open file paths in focused window

## Summary

Instead of opening file paths in a new window based on the file's parent directory, open file paths in the current window.

Also open folders in the current window when the current window doesn't already have a project path. This will affect untitled windows that were launched from the dock or by pressing `cmd-shift-N`.

You can still always force a new window from the command line by running `atom -n package.json` or `atom --new-window package.json`.

Multi-folder project support is still planned though via #770.

## File behavior changes

### Files opened using the `atom` command open in the currently focused window

![file-cli](https://cloud.githubusercontent.com/assets/671378/4016849/7fd61c08-2a39-11e4-830a-1feab5b7161c.gif)

### Dropping a file onto a window opens it in that window

![file-drop](https://cloud.githubusercontent.com/assets/671378/4016139/5a16e7de-2a31-11e4-8dfb-151f7b8c7dc1.gif)

### Selecting a file from the path chooser (cmd-o) opens that path in the current window

![file-open](https://cloud.githubusercontent.com/assets/671378/4016161/a415b130-2a31-11e4-9b30-34b494dda2e0.gif)

### Files dropped on the Mac OS X dock icon will open in the last focused window

![file-dock](https://cloud.githubusercontent.com/assets/671378/4016825/3a0815d2-2a39-11e4-939e-be6a3bd86e46.gif)

## Folder behavior changes

### Folders opened using the `atom` command will open in the currently focused window if it doesn't already have a project path

![folder-cli](https://cloud.githubusercontent.com/assets/671378/4016877/e013f6e4-2a39-11e4-806a-8bc004a0a567.gif)

### Dropping a folder onto an untitled window sets that window's project path to the dropped folder's path

![folder-drop](https://cloud.githubusercontent.com/assets/671378/4016898/2dbc2c36-2a3a-11e4-9000-464e11963ba6.gif)

### Selecting a folder from the path chooser (cmd-o) opens that folder in the current window if it doesn't already have a project path

![folder-open](https://cloud.githubusercontent.com/assets/671378/4016912/514a7522-2a3a-11e4-9197-38bdaf71fc5f.gif)

Closes #1722