Unbake the special help texts.

The text in the various help dialogs that was previously baked into an image is now drawn as text, meaning that it benefits from text unblurring when scaling is active.

This entails some small changes to layout, since it's a different font.

Dialog engine changes:
* A new picture type allowing to draw the inventory button icons directly into a dialog.
* A new widget type that simply draws a line between two points.

src/doxy/mainpage.md

@@ -192,6 +192,7 @@ only if `size` is `large`. Can be either `true` or `false`, defaults to
 `false`.
 * `filled` - For transparent graphics, specifies whether to draw a solid
 black fill behind the icon
+* `color` - The colour of the solid fill. Defaults to black.
 * `def-key` - See **Common Attributes** above.
 
 The possible values for the `type` attribute are: