detect a g layer with the className "Layer" instead of a non empty title

[cg-scorpio]

PR description

Currently in svgEdit, there is a title tag in all g elements that represent a svg layer. This causes an inappropriate tooltip to appear when the user hovers over a layer (see screen shot below).

To eliminate this behaviour, we have dynamically removed the title tags in the application that embeds SvgEdit (and therefore without modifying the SvgEdit code).

Without the title tags on the g elements that represent a svg layers, a bug occurred because SvgEdit detected a false problem when opening an svg: it thought that the drawing was not encapsulated in a layer and therefore decided to modify the svg and add a g element that encapsulates the drawing.

This behaviour is due to the way in which svg layers are detected: SvgEdit currently detects a svg layer by checking that it is a g element with a non-empty title tag. We suggest using another method to detect svg layer: check that it is a g element that includes the "layer" css class.

Checklist

Note that we require UI tests to ensure that the added feature will not be
nixed by some future fix and that there is at least some test-as-documentation
to indicate how the fix or enhancement is expected to behave.

• - Added Cypress UI tests
• [ X ] - Ran npm test, ensuring linting passes and that Cypress UI tests keep
  coverage to at least the same percent (reflected in the coverage badge
  that should be updated after the tests run)
• [ X ] - Added any user documentation. Though not required, this can be a big
  help both for future users and for the PR reviewer.