Add experimental options to style the 15505+ main menu popover to match stock Nickel (caret, separators, position, width)

[fyoosh]

Summary

On firmware 4.23.15505+, where the original main menu was removed, NickelMenu builds its menu from scratch as a bare NickelTouchMenu. Stock Nickel popovers (e.g. the brightness and battery popovers) draw a pointer caret aimed at their anchor, sit inset from the screen edge, and use full-width dividers — the from-scratch menu has none of that. This adds a handful of opt-in experimental: options to bring it closer to the native look.

Everything here defaults to the current behaviour, so an unset config renders exactly as before (the left screenshot above is this branch with no options set).

Options added

All under the existing menu_main_15505_* experimental namespace:

Option	Effect
caret	draw the native popover caret, aimed at the NickelMenu tab
decoration	caret edge (0 top, 3 bottom; 1/2 the side edges, 5 none)
caret_dx / caret_dy	nudge the caret anchor
offset_x / offset_y	nudge the popup position (e.g. a screen-edge margin)
width	fixed popover width (default auto-size)
flush_separators	full-width dividers via Nickel's own separator
separator_inset	inset for the above, to line up with the painted border
italic	0 renders item labels upright

Notes on the implementation

A few things were needed beyond what the existing code does, in case they're useful context for review:

• The caret only paints if the menu has a parent (drawDecoration returns early
  otherwise), showDecoration(true) is set (gated in paintEvent), and a target
  rect is given — so the menu is now parented to the tab button's window when the
  caret is requested.
• offset_x/_y are applied with move() after popup(), because popup()'s
  screen-fit clamping otherwise swallows them.
• flush_separators adds a LightMenuSeparator and widens it after layout,
  since the menu insets every action.
• italic 0 calls MenuTextItem::setTextItalics(false) after setText()
  (the constructor flag alone doesn't stick, as setText re-applies italics).

The four new NickelTouchMenu/MenuTextItem symbols are resolved optionally, so
firmware without them just falls back to the current appearance.

Testing

Built with nickeltc:1.0 and verified on a Kobo Libra Colour, FW 4.45.23697.
Symbol tags follow the existing entries (4.23.15505 *); CI will confirm across
versions.

A question on defaults

I've kept everything opt-in here to stay conservative — but I wanted to raise one idea:

The pointer/caret and full-width dividers are arguably less "a styling option" and more "the native Kobo look this menu is currently missing".

NickelMenu's menus look native because they are native widgets, and the 15505+ popover is the one element that doesn't (flat box, inset dividers, no caret/pointer) only because it's built from scratch rather than reused.

So — would you want the caret, full-width dividers, and a small edge margin on by default for the 15505+ menu, with width / italic staying opt-in? The caret anchor is derived from the tab button's own geometry, so it's device-independent, and the margin is just a small fixed inset; width and italic are the user-taste parts, which I'd suggest stay opt-in regardless.

No presumption either way — happy to leave it fully opt-in as in this PR, flip those to default, or whatever fits your appetite for a behaviour change. The "match native" angle just felt worth raising.

Thanks for your work on NickelMenu so far :)

Disclosure: I developed this with Claude Code (also noted in the commit
trailers). The reverse-engineering, build, and on-device testing were all
verified on hardware, and I'm happy to walk through any of it in review.