New canvas widget (initial version)

[dacap]

New canvas widget.

• paint filled rectangles

• paint images (Tiles) on canvas

• more mouse events, probably like HTML events:

  The order of events related to the onmouseup event (for the left/middle mouse button):

  1. onmousedown
  2. onmouseup
  3. onclick

  The order of events related to the onmouseup event (for the right mouse button):

  1. onmousedown
  2. onmouseup
  3. oncontextmenu

• paint theme elements on canvas

• access active theme colors

[dacap]

Some information in the next-version branch:
https://github.com/aseprite/api/blob/next-version/api/dialog.md#dialogcanvas
https://github.com/aseprite/api/blob/next-version/api/graphicscontext.md