Rework alignment
GtkAlign has four values:
- start
- right
- end
- fill
It's all good until you want to express a start or end alignment while the widget is filling all its allocation.
This is usefull mostly in transitions, e.g. to adjust the position of children in a non-homogeneous GtkStack.
AdwSqueezer and GtkLabel work around the issue by having xalign and yalign properties, to let their users refine how they want the content to fill its allocated space.
The heart of the issue I think is that filling isn't an alignment, it's at best its absence, and at worst a completely different concept.
What about dropping GTK_ALIGN_FILL and instead give GtkWidget a fill property? That would avoid the aforementionned workarounds.
Maybe we could drop GtkAlign completely and have a 0–1 value like for xalign too, if judged relevant.