Added support for specifying packages to import in library.properties (issue #2134)

Added support for specifying packages to import in library.properties. Searches for 'imports' as a key in library.properties. 

  imports=mylib, mylib.util

If found, these imports are added when the user selects 'Import Library'.  

  import mylib.&#42;
  import mylib.util.&#42;

If this line doesn't exist, falls back to current behavior and adds imports for all packages in the library's base jar. 

This involved one substantive change: passing the name of the library to JavaEditor.handleImportLibrary(String) rather than the jar path. Once the library is found (by name), then its jar path is available via Library.getJarPath().