Provide align hints to size_rules

[dhardy]

Re-work how alignment is handled: pass through size_rules, and do not prevent stretching by default. A new layout specifier, pack, is used to limit size and set alignment.

Motivation: #329 — alignment is now available when calculating size_rules, albeit only the input (or parent item's) alignment. The result here is clean enough code.

It is also possible to add alignment information to the returned SizeRules, thus exposing per-item alignment, however this leads to messier code (e.g. content alignment and widget alignment are not always equal, especially for Text content). Moreover, many widgets will prefer to stretch to all available space, thus making more refined combination of margins ineffective.

• Add field AxisInfo::align: Option<Align> for passing an align-hint into Layout::size_rules
• Remove AlignHints parameter from Layout::set_rect
• Add pack layout specifier to size-limit and align content
• Buttons are no longer automatically size-limited and aligned when not using Align::Stretch
• Stretch::None does not prevent stretching (see also Stretch model (usage of excess space) #349)
• Add AlignPair, replacing CompleteAlignment
• Text: SizeMgr::text_rules now sets alignment; ConfigMgr::text_set_size can optionally set alignment
• Slider widget: use ConfigMgr::align_feature
• Gallery: adjust scroll bars; pack theme-colour buttons
• Sub and SubAssign impls for Size now use saturating subtraction; remove Size::clamped_sub