FR: Support for templates (variables) in widget labels

[M0ebiu5]

Support for templates in each label field would be nice. Now its pretty extensive to compose a string with different variables for the label and this functionality is often needed.

eg Show door and lock status in a label: "Door is closed, lock is engaged". Definition maybe like "Door is {{label.doorstate}}, lock is {{label.lockstate}}".

It should be possible to update only one variable at once (Door is still closed, but lock status changed). The template should have a separate text field - so it would also be possible to enter default values in the "label" field (json).