Update direct edit of feature elements on diagram to exclude expression text

[pcdavid]

For example when invoking direct edit on an attribute with a default or initial value, displayed as "attr = 42":

• the initial direct edit text should exclude the "= 42" part;
• if the user enters a text which ends with "= something" or ":= somethingElse", this part should be ignored.

Expression edition should be available exclusively through the dedicated tool and modal.