Don't show autocomplete after backspacing over whitespace

[zarathir]

Fixes #743
Added EditCommand::DeleteBackward and EditCommand::DeleteForward to the match statement that checks for whitespace commands. On receiving one of these commands the deleted string gets checked if it's all whitespace.

Added EditCommand::Undo and EditCommand::Redo to the check, because i think it's not necessary to show autocompletion after an undo or redo.

Maybe it would make sense to extract this into a function?

[dzhou121]

You could use the slice_to_cow on the rope to make it more efficient without converting the rope to a string.

https://github.com/lapce/lapce/blob/master/lapce-core/src/buffer.rs#L931

[zarathir]

Would you pass the Rope to show_completion(...)? Because slice_to_cow takes a Range as argument it would end up something like this:

fn show_completion(cmd: &EditCommand, doc: &Rope, deltas: &[(RopeDelta, InvalLines)]) -> bool {
    [...]
    if start > &0 && end > start {
        !doc.slice_to_cow(*start..*end)
            .chars()
            .all(|c| c.is_whitespace() || c.is_ascii_whitespace())
    } else {
        true
}

I'll have to try if this works and update with results 😄