Implement save/load UI

[darkf]

Another dialog window. Includes a list of saved games along with name and date, and a mini screenshot of the game where it left off.

The latter should be easy by rendering the game canvas and exporting it to a data URI.

Related to #16.

[darkf]

Mostly done, just a few minor issues like text formatting and savegame list highlighting.

[darkf]

Highlighting is done. A few more issues remain:

• [x] Save item highlighting
• [ ] Formatting the list nicely, with appropriate padding/margins
• [X] The "Load/Save Game" text on the header
• [ ] "Empty." text in the terminal
• [ ] Save Screenshots
• [ ] Scrolling the save list
• [X] Save dialog (not just load dialog)

[darkf]

A few more of those were done; mainly screenshots and scrolling is left, the other things are very minor issues.

[darkf]

The existing save/load UI was reimplemented in the new UI lib as per #105. It should be easier to implement these features now.