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Co-authored-by: Rafael Siejakowski <rs@rs-math.net> Use GTKSourceView4 to implement syntax highlighting in the XML Dialog for CSS editing (both the contents of the <style> tag and inline styles) as well as for the SVG path `d` attribute and the `points` attribute of poly{line|gon} elements. Consistently-styled XML coloring is also used in the XML tree shown in the dialog, by recognizing XML elements and attributes. Several additional UI improvements are made to the XML Dialog: * When editing the `d` attribute, a number-rounding button is available. * The `d` attribute and inline CSS are automatically reformatted for better readability. * The attribute editing popover does not close on Enter press in these modes. Its resizing behavior and timings are improved. * The superfluous `svg:` namespace prefix is not shown in the XML tree. When the user creates a new node via the dialog, the prefix is accepted but not required. * A validation step is added for new el...
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