Allow SVG text tool to check whether it can edit the layer.

This moves some strings, but I wonder if this shouldn't be generalized more, so that @tymond can use it in the panel tool as well.

BUG:516001

Test Plan

1. Create a document with multiple vector layers.
2. Hide and/or lock some of those layers.
3. Try to use the text tool to add to each layer.
4. When the layer is hidden/locked, it should give a floating message.