This adds on_tool_activation() to Tool which GuideTool can use
to show guides, if they're hidden, when it's activated. Also
show guides on mousedown since there's no use in drawing invisible
guides.
This patch adds the logic for the GuideTool. Pulling in from outside the
image creates a new Guide, moving a Guide outside of the image deletes
it and a Guide can be deleteted via the context menu.
A Guide is considered clicked when the cursor is less than 20 pixels
away from the line.
