Task-number: QTCREATORBUG-2854

Reviewed-by: hjk

Reviewed-by: Erik Verbruggen

Reviewed-by: Thorbjørn Lindeijer

When the user types '(' then he'll likely also type the matching ')',
in which case skipping the automatic closing parenthesis is annoying
because you would end up with two of them.

Reviewed-by: Roberto Raggi

Groups are no longer enum values but identified from snippet providers.

Provide an interface so users can create/edit/remove snippets.

Reviewed-by: Roberto Raggi

Reviewed-by: Erik Verbruggen

In some cases snippets have a description. Due to the sorting this
already places them after keywords. So this is just to be consistent
for all cases.

Task-number: QTCREATORBUG-2307
Reviewed-by: Thorbjorn Lindeijer

Done with: Thorbjørn Lindeijer

Don't complete the current item when pressing special characters (e.g. `;') if the completion is automatically triggered.

Done-with: Erik Verbruggen

Don't automatically trigger the completion when editing a word or when
in a comment or string.

Allowing any length more than 2 is too slow at the moment. We'd first
need to find a way to avoid recalculating the complete list of items
for every character while the key isn't matching anything.

Now it's possible to choose between having completion popup manually,
when triggered or always.

Similar behaviour as for the QML editor. However, for now without
removing the completion box when you have typed the whole word, since it
could still be useful for automatically inserting other characters.

Task-number: QTCREATORBUG-67

Done-with: Erik Verbruggen

Now you can type characters like ., ( and : to complete the selected
item, depending on the type of the completion item.

Task-number: QTCREATORBUG-271
Reviewed-by: Roberto Raggi