Added examples-package as a new contribution type.

This fixes [#2582](https://github.com/processing/processing/issues/2582) and resolves [#2444](https://github.com/processing/processing/issues/2444).

In this branch, the following have been implemented:

* Added a new examples-package type, and the accompanying contribution manager.
* Added an "Add Examples" button to the Examples window, from where the Contribution Manager is to be accessed, as well as to the Open option of the Toolbar Menu
* Gave the UI of the Examples JTree a new look:

![examples_tree_new_look](https://cloud.githubusercontent.com/assets/4526417/3926675/8c6d7034-23f3-11e4-91e9-1bbc9de310b0.png)

* Prevented old backup folders from getting displayed in the mode/tool/library/examples-package sub-menus in both the Open option of the Toolbar Menu.
  
Note that for this mode, a new field "compatibleModesList" is required in the contributions.txt (for only example entries) and the examples-package.properties files. This will contain a list the fully quaififed classname of the mode that the examples-package is comaptible with (as can be seen from mode.id in the sketch.properties file).

#### Testing

To test this feature, replace [this](https://github.com/processing/processing/blob/master/app/src/processing/app/contrib/ContributionListing.java#L37) line with [this link](https://raw.githubusercontent.com/joelmoniz/G4P_Examples/master/contributions.txt). Then, switch to Java (2.0) mode and go to file>examples>Add Examples... and install the contribution that displays there.
@joelmoniz Is this necessary? It looks ugly on Windows. Probably a JButton would be better here? I could open a PR to fix.