Add support for optional text completion on textfield widgets

[pcdavid]

When editing text fields which are not "free form" but correspond to some constrained DSL, it would be nice to have support for completion proposals (computed on the backend). For example: AQL expressions or domain types in view definitions.

This should be optional, only enabled on Textfield widgets which explicitly provide a completion proposal provider. Ideally, there should be some visible affordance on such widgets to tell the end-user that completion is available on this widget.

The user-experience should be globally similar to what is customary in editors and IDEs like VS Code or Eclipse, but without all the bells and whistles:

• triggered on some key combination (Ctrl+Space if possible), which sends a request to the backend;
• upon receiving the possible completion proposals from the backend, open them in a list at the cursor's position;
• if the user selects one of the proposals (using the mouse or, ideally, using the arrow keys), insert the proposal's text.