Arbitrary path support for checks' and overview stats
Provide qualitycheck category names to all editor instances
Added convenience property
FOLLOWUP: Fix previous commit
We won't always have a resource path
Use `code` for identifying all browser items

This will allow to handle all kind of browsable objects.
`hasattr` on a dict? No way!
Clarify the difference between context and resource objects
Don't return iterators as children objects

Since we need to access children elements several times, we better keep
them in memory and avoid the need to rewind iterators.
Refactored `get_name` to identify objects by code rather than by name
Implemented `get_cache_key`

Also enforced children objects to implement it.
Cleanup: removed dead code
Cleanup: coding style fixes
Cleanup: removed dead code
Organized TreeItem overriden methods
Flush cache before stats will be refreshed
Add flush_cache PootleCommand
Retrieve check failures for resource objects
Display check failure widget when accessing via URL
Cleanup: coding style fixes