Convert Glyph Layers to SVG Font: Traceback when font includes apostrophe (single-quote) glyph.

Steps to reproduce:

• open Inkscape
• Select Extensions > Typography > 1 - Setup Typography Canvas
• Select Extensions > Typography > 2 - Add Glyph Layer
• Enter ' as the unicode character; click Apply.
• Select Extensions > Typography > 3 - Convert Glyph Layers to SVG Font

OR

• open Inkscape
• Open an existing SVG font document that contains a glyph for ' (sometimes encoded as '). For example, svg_fonts/HersheySans1.svg or the SVG version of Relief SingleLine.
• Select Extensions > Typography > Convert SVG Font to Glyph Layers
• Select Extensions > Typography > 3 - Convert Glyph Layers to SVG Font

What happened?

Traceback:

Traceback (most recent call last):
  File "/Applications/Inkscape.app/Contents/Resources/share/inkscape/extensions/layers2svgfont.py", line 102, in <module>
    LayersToSvgFont().run()
  File "/Applications/Inkscape.app/Contents/Resources/share/inkscape/extensions/inkex/base.py", line 231, in run
    self.save_raw(self.effect())
  File "/Applications/Inkscape.app/Contents/Resources/share/inkscape/extensions/layers2svgfont.py", line 68, in effect
    glyph = font.get_or_create(
  File "/Applications/Inkscape.app/Contents/Resources/share/inkscape/extensions/inkex/elements/_base.py", line 361, in get_or_create
    node = self.findone(xpath)
  File "/Applications/Inkscape.app/Contents/Resources/share/inkscape/extensions/inkex/elements/_base.py", line 445, in findone
    el_list = self.xpath(xpath)
  File "/Applications/Inkscape.app/Contents/Resources/share/inkscape/extensions/inkex/elements/_base.py", line 435, in xpath
    return super().xpath(pattern, namespaces=namespaces)
  File "src/lxml/etree.pyx", line 1597, in lxml.etree._Element.xpath
  File "src/lxml/xpath.pxi", line 305, in lxml.etree.XPathElementEvaluator.__call__
  File "src/lxml/xpath.pxi", line 225, in lxml.etree._XPathEvaluatorBase._handle_result
lxml.etree.XPathEvalError: Invalid predicate

What should have happened?

The extension should operate (without producing a traceback).

Inkscape Version and Operating System:

• Inkscape Version: Inkscape 1.2 (dc2aeda, 2022-05-15), with latest extensions from master.
• Operating System: macOS Monterey
• Operating System version: 12.3.1

Root cause and solution:

The relevant code in layers2svgfont.py reads as follows:

"svg:glyph[@unicode='{}']".format(unicode_char)

When the value of unicode_char is ', this reduces to "svg:glyph[@unicode=''']" which is problematic.

I'll follow up with a merge request shortly.