Add missing shell methods on atom global

[savetheclocktower]

Closes #1563.

This adds in the following methods:

• atom.trashItem
• atom.openPath
• atom.openExternal
• atom.showItemInFolder

All of these exist on require('electron').shell, but are not 100% able to be called from the renderer for one reason or another. As with many other methods defined on the atom global, these methods proxy to the main process via IPC, then receive the outcome from the main process.

Specs are present for atom.trashItem, but I've got no clue how I'd test the others. That'll require manual testing.

[Daeraxa]

I tried them all out in the dev tools console and they seem to work fine. Even substituted the shell.trashItem in my init script for atom.trashItem and it is still working.

Also checked that atom.trashItem still honours $ELECTRON_TRASH, was able to set that to kioclient and watch it fail in the same way as if it was set on shell.trashItem.