Add syntax coloring to XML dialog

Add the Gtk Source View library to enable syntax coloring in the XML Dialog when editing style and d attributes. This MR is a collaboration with @S-Rafael.

Inline-style and path "d" attribute are reformatted for editing (aka pretty-printed) making it easy to see individual elements (styles and path commands).

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Coloring applies to XML dialog tree too:

The tree no longer shows the superfluous svg: prefix for SVG namespace elements.

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Syntax coloring for <style> content:

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Syntax highlighting scheme can be selected in the preferences. Apart form stock themes that are two Inkscape-specific themes for light and dark UI theme, that are created specifically for our use of highlighting. They support in-line CSS styles and SVG path "d" attribute fully. Other themes may not offer distinct colors for all path command arguments.

Those two themes are also special in a sense that Inkscape can automatically select correct one (that is dark or light) based on current UI theme.

• Theme selector
• Ability to turn coloring off
• Option to use monospaced font in XML dialog

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Improved experience adding new element in XML dialog:

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To do

• Ensure that CSS minification doesn't remove semantic whitespace (don't strip spaces inside parens).
• Add a language definition for the SVG d attribute.
• Implement reformatting for d.
• Force style/path d attribute editing in a popup.
• Apply color scheme to XML tree.
• Add user preferences for selecting themes.
• Add syntax coloring for <polygon> points.
• Add gtksourveview4 as a dependency in the CI pipeline: see inkscape-ci-docker!34 (merged)
• Ensure the gtksourceview library is bundled in the Windows installation.
• Ensure it's also packaged into the AppImage (should be taken care of by inkscape-ci-docker!34 (merged))
• Ensure it's also packaged into the macOS application.

Stretch goal:

• Add warning for invalid SVG path data attribute.

Closes inbox#4263 (closed) | Closes inbox#4264 (closed)