Improved the TreeItem delegate which simplifies making tree items:
- tree items can define any number of key/value pairs
- creating a tree you specify which columns you want to display and it will pick out the right key/value pairs from the new tree item dictionaries
- added new "tk-target" command to explore the target's images, sections, symbols, compile units and line tables.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@219219 91177308-0d34-0410-b5e6-96231b3b80d8