Prevent the number of columns to do below 12 in demo

[jccampagne]

This is a quick fix for the demo as it would crash if the DragValue went below 12.

This is because the groups are hard coded as:

egui_table::HeaderRow {
    height: self.top_row_height,
    groups: vec![0..1, 1..4, 4..8, 8..12],
},

This is really a minimal fix, to prevent the demo from crashing (wasm and native).

I think it's more a bug of the demo rather than an issue with the crate, hence this quick fix rather than a more extensive-logic change in the demo...

The app crash in

egui_table::table::TableSplitScrollDelegate
fn header_ui(&mut self, ui: &mut Ui, offset: Vec2)

because groups would become stale, because of how their are calculated:

            let groups = if !header_row.groups.is_empty()
                && header_row
                    .groups
                    .iter()
                    .all(|g| g.end <= num_columns)
            {
                header_row.groups.clone()
            } else {
                (0..num_columns).map(|i| i..i + 1).collect()
            };

• Closes index out of bounds panic when modifying table demo column count #23