Implemented Feature to Add/Remove Graph Documents from the Project.

This request resolves two issues:

1. Prevents setting Graph Document Names to empty strings.
2. Implements feature to add/remove graph documents, based on this issue.

I have provided a "Select Graph Documents" Menu Option in Graph Documents, which opens up the dialog box for the user to select Graph Documents from.

The following new documents have been created, to implement the Dialog Box:

• ui/selectgraphwidget.cpp
• ui/selectgraphwidget.h
• ui/selectgraphwidget.ui

Here are some issues that I'm aware of, and need advice on:

• The signals and slots responsible for opening and closing the tabs requested by the user have been named myActiveGraphDocumentChanged and onMyActiveGraphDocumentChanged respectively. A different name should be used, because there already exist signals ActiveGraphDocumentChanged, and it could cause confusion.

• The method responsible for validating user inputs in the Dialog Box, has been named setValues(), and it could also have a better name, I feel.