Allow toggle completion instead of autocompletion

[netzmensch]

It would be nice to be able to actively tell the plugin "I want an autocompletion for the current line" instead of "do autocomplete all the time" with a shortcut.

So instead of query the model after 250ms automatically after the last pressed key, just wait for the user to press a shortcut.

[scscgit]

There is also the opposite reason: it often doesn't start autocompletion at all until you type some random characters. You don't always know the first character of the code that should be autocompleted.