Add polygon tool

[ZMYaro]

• [x] First 2 points must be added in one pointer action (like using the line tool).
• [x] Additional points can be moved while pointer is down.
• [x] If in fill only mode, first line inverts the colors below like freeform selection (this matches the behavior from classic Paint; Win7 Paint does not show anything, which seems like a worse UX).
• [x] Clicking near the first point closes and finalizes the polygon. (Experimenting in classic Paint, it seems to close when within 5px of the starting point for a 1px line.)
• [x] Double-clicking closes and finalizes the polygon.
• [x] Switching tools closes and finalizes the polygon.
• [x] Esc cancels the polygon.
• [x] Add tool to help dialog
• [x] Add Esc shortcut to keyboard shortcuts dialog.
• [x] The color of the draft polygon should change immediately upon either of the current colors changing.
• [x] Add all of the above interactions to the comprehensive test plan.

[ZMYaro]

Generally implemented in https://github.com/ZMYaro/paintz/commit/545f9165b4c99bd30a528f3f3ba946dfc0c56647. Inverted color outline for fill-only mode and double-click to close still to-do.

[ZMYaro]

The tool is essentially done now, but I am leaving this ticket open because I think the icon needs to be moved up a pixel or so, and the left part needs to be slanted more to make it more visually balanced horizontally.

[ZMYaro]

^ Addressed in https://github.com/ZMYaro/paintz/commit/d8740fa7b16462a13262687a21683e77ba0ecf7f.