boxplot tick labels

The parameter labels has been removed in favour of tick_labels for clarity and consistency with bar.

Image path semantics of toolmanager-based tools

Previously, MEP22 ("toolmanager-based") Tools would try to load their icon (tool.image) relative to the current working directory, or, as a fallback, from Matplotlib's own image directory. Because both approaches are problematic for third-party tools (the end-user may change the current working directory at any time, and third-parties cannot add new icons in Matplotlib's image directory), this behavior has been removed; instead, tool.image is now interpreted relative to the directory containing the source file where the Tool.image class attribute is defined. (Defining tool.image as an absolute path also works and is compatible with both the old and the new semantics.)