Implement Windows Explorer "Open with..." option

### Overview
This change implements the [Edge Code](https://trello.com/c/U7pQ0yIg/117-open-with-brackets-option-from-windows-explorer-5) and [Brackets](https://trello.com/c/GhF1Bgf9/280-5-open-with-brackets-option-from-windows-explorer) user stories that allow Brackets to support the Windows Explorer "Open With..." command.

Please note that we still need an installer change to (1) pre-populate the necessary Registry entries prior to first launch; and (2) clean up the ProgID entries on Brackets uninstallation.  However, even without these installer changes, the "Open With..." feature will still work, so long as you launch Brackets at least once before attempting to "open with..." a file.

### Example Workflows
1. install Brackets (or launch Brackets at least once)
2. launch Windows Explorer
3. browse to and select a supported file (eg. a .js file)
4. right-click and choose "Open With..."
5. select Brackets.exe from the resulting context menu or dialog (note: depending on whether other applications have also associated themselves with this file type, you might either see a dialog or context menu from which to select Brackets.exe)
6. one of the following occur, depending on whether Brackets is already running:
   * if Brackets isn't already running, launches Brackets, opens selected file, and adds it to the working set;
   * if exactly one instance of Brackets is already running, opens selected file in that instance of Brackets, and adds it to the working set; or
   * if >1 instance of Brackets is already running, opens selected file in the top-most (ie. most recently used) instance of Brackets, and adds it tot he working set.

[note: Brackets Sprint 28 already supports opening a single file from the Brackets command-line.  However, it always launches a new instance to open the given file.]

### Implementation
This Windows-only, brackets-shell change adds two, new pieces of functionality:
   1. opens given file in the most-recent running instance, rather than launching a new Brackets window; and
   2. adds initialization/update of file association entries in the Windows Registry

For the first item, I've added a mutex to track when a Brackets instance is already running.  Then, when a new file is opened from the command-line, the new instance will enumerate any already running instances of Brackets, starting with the top-most window instance (aka the most-recently used window), bring it to the front of the desktop windows, and send it the filename via a WM_COPYDATA message.  Then, the second instance will immediately exit.  Upon receipt of the WM_COPYDATA message, the receiving instance of Brackets will then call existing Javascript code to open the file and add it to the working set.

For the second item, I've added initialization code to create/update Registry entries corresponding to those associated with an application ProgID and individual Explorer FileExts entries in support of the "Open With..." command.  This extra measure of initialization/update will help keep Brackets up-to-date, if for example you are running different builds of Brackets, without requiring that you uninstall and reinstall the application in order to continue using the "Open With..." command.