Added support for specifying packages to import in library.properties (updated version of PR #2178)

This is the updated, auto-merge-able version of PR #2178. It closes PR #2178 and fixes #109.

(The following is taken from the original PR, #2172:)
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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.*
import mylib.util.*

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().