Add item "Insert Image..." to edit menu

add new menu item which shows the ImageToolDialog
Extend ImageToolDialog class

* Add methods to return user input
* Add button to choose an image file from filesystem
* Add copyright to sources
Add insertImageLink() method

Add method to insert an image link into the markdown document to the
MarkdownManipulator class.
Insert image link into markdown document

Use new MarkdownManipulator::insertImageLink() method to insert an
image link to the markdown document based on the user input in
the ImageToolDialog.