Widget status tracker and method shims

[dhardy]

The widget API makes some requirements of the call order of methods.

Should we add a tracker to enforce this (in debug builds)?

Rules

Rules (x: y → z means that after x changes/happens, y must happen before z):

• init: configure → update
• init: configure → size_rules
• init: size_rules (x-axis) → size_rules (y-axis)
• init: size_rules (y-axis) → set_rect
• init, resize: set_rect → find_id
• init, resize: set_rect → draw
• init, data change: update → draw
• size_rules: set_rect → draw (maybe?)
• init: configure → nav_next (maybe?)
• init: configure → handle_event (and other handlers)

Failure to respect these rules can cause issues, e.g. invalid WidgetId (due to something before configure) or out-of-range in MatrixView due to set_rect then draw before the requested re-configure.

Proposal

Add an enum like:

pub enum WidgetStatus {
    New,
    Configured,
    SizeRulesX,
    SizeRulesY,
    SetRect,
}

(Note: this omits "data change" since the widget cannot reliably detect this.)

Add a status: WidgetStatus field to widget_core!() (in debug builds).

Add shims around all relevant widget methods which test and update this status. Problem: we need a solution to #407.