Added delete project action to the context menu of the project. Fixes #17
Edited the misleading caption in the global settings.
Refactored ProjectPropertiesManager.
Now it is simpler add arbitrary defaults for project properties.
Source and Target levels are now available in the models.
It is now possible to rely on the build script for sourceCompatibility and targetCompatibility.
Organized imports.
Adjusted the project node creation and finding children. This provides better integration with the findbugs plugin  in the expense of possibly creating the project node on the EDT.
The file "gradle.properties" is now displayed in the project view.
The user home directory for finding the sources and binaries of artifacts is the one specified in the global settings.