Start decoupling Toolbox elements from XML (sections+labels).

Just a small tweak and some tests to verify these things can indeed be created from dictionaries. Also slightly simplifies galaxy.tools.
Add commnt to auto-generated integrated_tool_panel.xml.
Simplify toolbox.load_tool_tag_set method.

guid is taken as input but is always produced the same way from the same input, so just derive it inside the method instead.
Introduce ToolPanel abtraction.

There are some repeated operations on an odict in Toolbox that could be cleaned up (fewer lines of code, more readable) with new abstraction in place. Starting with __add_tool_to_tool_panel but will work its way to other methods in Toolbox eventually.

Abstraction reduces cyclomatic complexity of Toolbox.__add_tool_to_tool_panel from 23 down to 18.
Introduce new abstraction of tool lineages.

 - Don't force non-toolshed tools into the tool shed install database.
 - Abstract out dealing with the tool shed install database from toolbox, to potentially allow other tool lineages implementations.
Hide details of toolbox.tools_by_id from lib/tool_shed/util/tool_util.py.
Centralize abstraction for removing tools from toolbox.

Shield's toolbox.tools_by_id from lib/tool_shed/galaxy_install/tools/tool_panel_manager.py.
Abstractions for registering tools from outside toolbox.

Part of effort to further limit exposing tools_by_id directly outside Toolbox.
Introduce abstraction to prevent lib/galaxy/workflow/modules.py from needing tools_by_id.
Eliminate direct in/get checks against toolbox.tools_by_id - use get_tool or has_tool.
Eliminate walking over tools_by_id externally from toolbox.
Move more logic into ToolSectionElements, raising level abstraction in Toolbox.
Replace another repeated idiom in Toolbox with higher level method in toolbox.panel.