Currently when a specifier enters an interpreted expression in the VSM editor, we only provide a single-line text field. Even for Acceleo 2, it is sometimes too small to be readable, and Acceleo 3 is more verbose than Acceleo 2. We need to provide support for entering long (including multi-line) expressions from within the VSM editor.
One possibility is to add a button labelled Edit... at the right of every interpreted expression text field. When clicked, it would open a simple dialog box with a multi-line text area initialized with the current value of the expression, so that the specifier can edit it more confortably. The dialog box could provide a combo where the user can select the language to use (Acceleo 2, Acceleo 3, OCL, plain text...), and some languages/interpreters could provide a richer editor (for example with syntax coloring and completion). Acceleo 3 could reuse the widget used to enter expressions in the Requestor view.
