Content in a PDF's annotation layer, such as link annotations, can
interfere with text selection and also cause creating annotations to
fail [1]. This commit resolves the problem by temporarily hiding annotation
layers when there is a non-empty selection. This allows the user to
select text inside link annotations (for example) while still enabling
the user to activate the link normally when there is no selection.

Fixes https://github.com/hypothesis/client/issues/1464

[1] This is because the annotation layer is outside the text layer and
    the PDF anchoring code expects the selection to be within the text
    layer.