Some tools (e.g. ZoomTool) doesn't need layer to work. This commit
makes mouse events fire even if there is no layer. This fixes
a bug that ZoomTool didn't work when there is no layers.
This commit adds a Tool::MouseEvent struct, which contains events that
may be needed by tools: layer-relative, image-relative and raw (editor-
relative) event.
The raw event is used by ZoomTool to properly pan the view. This fixes
a bug which caused image to snap out of sight.
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 adds support for the Tools in PixelPaint to use different cursors
within ImageEditor. For now most of them get the crosshair cursor since
it's the most fitting, but in the future we will want to add custom
cursors.
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.
