Use libpanel for grid/layout
To port Builder to GTK 4 and allow reuse with Drafting, libpanel was created (building up on libadwaita). We should drop IdeGrid, IdeGridColumn, etc and use libpanel instead.
We can look at static linking it though, since it isn't meant to be used from libraries, only applications. That means that something like Drafting would not use it in libdrafting, but the drafting binary could. Following this convention would mean we can use it statically without GObject Introspection issues.