[Feature] Pane Grid: drag & drop panes to the edges

[jhff]

Summary

Previously, in order to create a new column or row on a PaneGrid, a client would necessarily have to create and remove panes, as it was only able to split existing panes.

Following #1856, this change adds the ability to drag & drop panes of a PaneGrid to its edges in order to enhance even more an easy layout customisation of the grid.

Implementation

DragEvent::Dropped instead of Pane and Region parameters, now holds a Target that can either be Target::PaneGrid (new variant, that wraps an Edge) or Target::Pane (which holds the previous parameters).

Users of PaneGrid can call pane_grid::State::move_to_edge(pane, edge) in order to create a new row/column depending on the edge with that pane's state. This will split the major Node of the grid in order to create said row/column.

Hovered PaneGrid edges are detected and highlighted with the minimum thickness of either the total grid height or width ratio of 1/25. I feel it is a good balance after some experimentation, but I'm up to change this.

Demo

pane_grid_edges.mov

[hecrj]

Awesome stuff! Thanks 🥳

Made some very minor changes here and there and added a new drop helper method to State.

Let's ship this! 🚢