Edit › Stroke: a line along the selection's outline

[rezaularif]

Adds Edit › Stroke, which draws a line along the selection's outline, as in Photoshop. Closes #177.

What's added

• Edit › Stroke… with Width, Color (foreground or background), Opacity and Location (Inside, Center or Outside).
• On a mask, it paints black or white.
• Only the tiles the line passes through are touched, so a thin stroke round a large selection stays quick.

How to use

• Make a selection first: Marquee (M), Lasso (L), Magic Wand (W), or ⌘-click a layer's thumbnail to select its pixels.
• With one visible layer or mask selected, choose Edit › Stroke…, set the options and click OK.
• The menu item stays grayed out while there is no selection, or when two or more layers, a folder, or an adjustment layer is selected.

Tests

• SelectionStrokeTests. The full suite passes, and the project format is unchanged.