Fix a bug about drog-and-drop

*Bug Description:
If there is a cloud application (named 'sshsample' for example) in the applicationViewer, it also has no relation with any local project in the IDE. 
![draganddropanprojectwiththesamenamewithacloudapplication](https://cloud.githubusercontent.com/assets/7316500/4006332/71af4820-29a8-11e4-96bc-e2fd9dc42aae.PNG)

And then dragging and dropping a project which has the same name 'sshsample' to the applicationViewer, will cause the existing cloud application 'sshsample' in CF to be deleted, and the new published application 'sshsample' is shown with 'Local module is not yet deployed. Cannot refrsh with server'

![draganddropcausethecloudapplicationtobedeleted](https://cloud.githubusercontent.com/assets/7316500/4006543/87b5025e-29ad-11e4-81f8-85b10d4e603b.PNG)

*Bug Cause:
After the new project dragged and dropped into the applicationViewer,  ApplicationViewersDropAdapter.super.performDrop(data) is called, in which a new Module instance with the project name is generated in the Server(It is an instance in IDE to manipulate applications in the particular space of an orgnization with a specific account in CF). 


Then, CloudFoundryServer.updateModules() is called to refresh the ApplicationViewr. During this process, server.getModules() is called which returns the list of all existing modules (the newly generated module 'sshsample' will be the first element). The following loop 'for (IModule module : server.getModules()) '  will add the CloudFoundryApplicationModule with the same name 'sshsample' to 'deletedModules'. The modules in 'deletedModules' will be deleted from the server, which will trigger the deltion of the corresponding cloud application hosted in CF. The details of this process can be seen in https://github.com/cloudfoundry/eclipse-integration-cloudfoundry/blob/master/org.cloudfoundry.ide.eclipse.server.core/src/org/cloudfoundry/ide/eclipse/server/core/internal/CloudFoundryServer.java Line 644-732.


*Bug Fix:
The module generated and refresh process is quite complex, but the fix can be achieved easily from the drag-and-drop operation. During the original drag-and-drop operation, we firstly check whether there is an existing CloudFoundryApplicationModule with the the same name as the drag-and-drop project. If it does exist, we show a message dialog to inform the user that the current project can't be drag to the applicationViewer and return without adding the dragged project to applicationViewer.

![draganddropbugfix](https://cloud.githubusercontent.com/assets/7316500/4006594/9225eb8e-29af-11e4-805e-772ffed8c37a.PNG)