Added Edit menu with Cut, Copy, and Paste items

Instead of adding a popup menu to the Account Dialog, I have added an Edit menu with Cut, Copy, and Paste functions. These features work with any text field in the Account Dialog. This allows users to cut, copy, and/or paste to/from UPM to/from any other application. This addresses the feature request from Issue #24 "Cut&Paste in Notes textarea". I think this feature would be useful for users who are manually migrating from another password manager, such as myself. I know cut, copy, and paste functionality is inherent with text fields via keyboard shortcuts (i.e., Ctrl+X, Ctrl+C, and Ctrl+V), but not all users are familiar with or are used to using keyboard shortcuts. I know my wife instinctively looks for Edit->Paste when pasting text into applications.

Furthermore, I have done a lot of clean-up of the code, such as cleaning up indentations and code alignment, and cleaning up comments throughout the Account Dialog file. Some of the comments were referring to the wrong method. Example: Some comments mentioned, "The above method..." where the actual method being described was below the comment.

If this is something you think could be added to UPM, here is my pull request.