Tables and aligned menus

[dhardy]

KAS already supports:

• List: a row or column of widgets
• Grid: a "matrix" layout of widgets, supporting merged cells
• ListView: a list of user-typed data (ListData trait) with a Driver creating/reading/writing temporary view-widgets over each data item
• MatrixView: like ListView, but with grid layout and 2-dimensional data array (MatrixData)

Tables are a little bit different:

• rows and columns should support titles; this could be added to MatrixView as an optional feature
• each column should have its own data type

The latter is trickier to support. Some suggestions:

1. We could make the data trait and view widget specific to the number of columns, allowing a type for each column. This is the simplest to implement, but then we need many traits and widgets.
2. We could use the MatrixData trait with type Box<dyn Any>, then downcast data when reading, somehow.
3. We could use the ListData trait where the data type is a tuple representing a column, and construct one widget per row (like examples/data-list-view.rs), but then we need some hacks to ensure that columns and titles get aligned ("interior alignment", a complex feature), and to allow per-element selection.
4. Again, we could use the ListData trait with a tuple per row, but use a variant of the Driver trait which either operates on a list (whole row) of widgets at once or uses a column-index. Like (2) we would need to downcast view-widgets to read/write, but at least this option avoids having to downcast data.

The last option appears the best, but still needs a specific TableView widget
and TableDriver trait.

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Why this applies to menus: they are essentially tables; e.g. KDE's menus have columns for a checkbox, icon, label, shortcut, and sub-menu indicator. (Or they could just be a row with label, filler, and extra content.)

[dhardy]

Additional features:

• Odd/even row highlighting: pass the row number to the theme.
• Trees: this can just be a column to show + / − marks and a row-filter (so long as we don't need to animate the transition).
• Visual column/row dividers: just a border of theme-specified size. If this is simply drawn as lines/rects then drawing is simple, otherwise row/column intersections might be complex — but dividing lines should be small, so that's an unnecessary detail. Potentially selection boxes can be drawn on top.

[dhardy]

Menu item alignment was done in #305.