Add the ability to open files

[jamespfluger]

eXeMeL already allows auto-loading content from the clipboard, and saving the contents to an XML file. However, it does not allow deliberate opening of a file.

This can be easily implemented by adding a command extremely similar to SaveCommand_Execute, but using OpenFileDialog instead of the save. Additionally, the OpenFileAsync method already exists and can be called from within. However, that method cannot be awaited because it returns void.

Things to consider:

1. Is it worth creating a new non-async method for OpenFile? The code would be nearly identical, so I don't think it makes sense.
2. What icon should be used for the open command? The appbar.folder.open icon from the source of the rest doesn't look great because of how small it is. This assumes no additional resizing happens.
3. Where does the icon fall in the toolbar? Open is traditionally to the left of Save, but all the buttons are placed on the right side.

I've already got a working version of this, but I want to refine it some more. And finalize the items listed above.