The Input suggester should only show suggestions if an active edit is happening

[davep]

Somewhat following on from #3806, I feel that the suggester API for Input is a wee bit too eager to make a suggestion. Consider the following code:

from textual.app import App, ComposeResult
from textual.suggester import SuggestFromList
from textual.widgets import Input

class SuggestTooSoon(App[None]):

    def compose(self) -> ComposeResult:
        yield Input("TeX", suggester=SuggestFromList([
            "textual"
        ], case_sensitive=False))

if __name__ == "__main__":
    SuggestTooSoon().run()

imagine this is a field in a dialog where the Input has been populated from some external database. The value in the field is the correct value. On the other hand the current approach suggests it might not be:

The proposal here is that we modify the suggester API so that it doesn't make a visible suggestion until the user makes an edit. The "correct" value in that Input is TeX -- no suggestion needed -- but the moment the user backspaces the X, or adds a t, then it makes sense that a suggestion is made.

It may be that the current behaviour is desirable in some situations, in which case it would be fine to make this uneager approach an option.

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