[UX] have AdminVM manage inter-vm file copying

[mfc]

this came out of a discussion with @bnvk:

when copying a file/folder, Instead of source qube1 popping up a window where one types in the destination qube2 name, which is then confirmed via AdminVM, why not have a dialog in AdminVM pop up that allows one to have a drop-down of available VMs, and even available programs within those VMs?

Current workflow:

1. right-click on file, select Copy/Move To VM
2. source qube1 dialog box: user types in destination qubes2 name
3. AdminVM dialog box: confirm file transfer

Proposed workflow:

1. right-click on flie, select Copy/Move To VM
2. AdminVM dialog box: select destination VM (confirmation of file transfer is implicit)

This would also remove the spamming of source qube1 dialog box if one has selected multiple files to copy.

[marmarek]

This is duplicate of #910
And yes, generally IMO good idea.