Add a way to select and display a background image

This is a rough attempt at the feature mentioned above.
Refactoring of background image feature

Moved code out of paintComponent method and made the background image to be cleared when the cancel button is clicked.
Merge branch 'master' of https://github.com/freeplane/freeplane
Fix method of centering background image
Add action to clear background image
Minor Refactoring
Code Formatting Adjustments
Code Formatting Adjustments on MapView
Code Formatting Adjustments on ViewerController
Code Formatting Adjustments for MindMapModeMenu
Further Code Formatting Adjustments for MindMapModeMenu
Modify way of retrieving the background image URL
Refactor rendering method of background image
Refactor method of retrieving URI of background image
Refactor rendering method of background image
Minor fix on how the error message is printed
Code Formatting Adjustments on MapBackgroundImageAction
Refactor MapView
Add SVG support (in progress)

The image is rendered on top of the nodes. Moreover, I have yet to address the other issues mentioned.
Add functionality to draw viewer component behind all nodes

As Dimitry pointed out, I called the viewer component's paint method directly from MapView's paintComponent method. Moreover, I did some minor refactoring. In NodeView's paintComponent method, there were two switch statements in a row with the same switch expression, but is there a specific reason for that? (I merged them into one).