New canvas widget (initial version)

[dacap]

New canvas widget.

• [x] paint filled rectangles
• [x] paint images (Tiles) on canvas

• [x] 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

• [x] paint theme elements on canvas
• [x] 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