Add filename autocompletion to Open/Save dialog

[haoruilee]

Implements filename autocompletion in the Open/Save file dialog (#28).

Features:

• Shows suggestions as users type in the filename field
• Tab to select top suggestion
• Provides mouse click selection of suggestions
• Limits to 5 suggestions maximum

Closes #28

[lhecker]

I tested this locally and this is really awesome!

I think we should move the floater to be below the editline. This is because there may not be a lot of space above the dialog.
We could also hook arrow up/down by calling consume_shortcut before the call to editline and deferring focus to the floater if it's currently open. This would then make it very comfortable switching focus into the floater.

What do you think? (Please feel free to disagree.)

[haoruilee]

Looks better now:

1. Move the floater below the editline
2. Switch from editline to floater using ↑/↓, if at the top one then ↑ back to the editline
3. Highlight in floater

[DHowett]

I CANNOT WAIT to use this. Thank you!