* Tooltip can be added to any target widget (e.g. Button) and pop up on
mouse over.
* Refactor the event system. Instead of passing a render.Point to all
event handlers, pass an EventData struct which can hold the Point or
the render.Engine.
* Add event types Computed and Present, so a widget can set a handler on
whenever its Computed or Present method is called.
* Toolbar has icon buttons for the Pencil Tool, Line Tool, Rect Tool,
Actor Tool and Link Tool.
* Remove the tab buttons from the top of the Palette window. The palette
tab is now toggled between Swatches and Doodads by the tool selected
on the tool bar, instead of the tab buttons setting the tool.
* Remove the "Link Doodads" button from the Doodad Palette. The Link
Tool has its own dedicated toolbar button with the others.
Since SDL2 is using in-memory bitmaps the same as Canvas engine, the
function names of the render.Engine interface have been cleaned up:
* NewTexture(filename, image) -> StoreTexture(name, image)
Create a new cached texture with a given name.
* NewBitmap(filename) -> LoadTexture(name)
Recall a stored texture with a given name.
* level.Chunk.ToBitmap uses simpler names for the textures instead of
userdir.CacheFilename file-like paths.
