Update and reorganise themes (#893)

* Drop the subfolders for GUI themes
* Remove bundled font support
* Add safe file read function
* Allow loading themes from multiple locations
* Clean up unused variables
* Clean up icons class
* Merge icon theme icons
* Update icon theme class
* Remove guiDark config setting
* Fix tests
* Fix handling of default theme
* Update icon readmes
* Add test for getGuiItem common function

novelwriter/assets/icons/typicons_dark/typ_folder.svg

@@ -0,0 +1,35 @@
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+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   version="1.2"
+   width="24"
+   height="24"
+   viewBox="0 0 24 24"
+   id="svg2765">
+  <metadata
+     id="metadata2771">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
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+  <path
+     d="M 18.666667,5.2499999 H 12 C 12,4.008 11.004444,3 9.777778,3 H 5.3333333 C 3.4955556,3 2,4.51425 2,6.3749999 V 17.624999 C 2,19.48575 3.4955556,21 5.3333333,21 H 18.666667 C 20.504444,21 22,19.48575 22,17.624999 V 8.625 C 22,6.76425 20.504444,5.2499999 18.666667,5.2499999 Z m -13.3333337,0 H 9.777778 c 0,1.2420001 0.995555,2.25 2.222222,2.25 h 6.666667 c 0.613333,0 1.111111,0.504 1.111111,1.1250001 H 4.2222222 V 6.3749999 c 0,-0.6209999 0.4977778,-1.125 1.1111111,-1.125 z M 18.666667,18.75 H 5.3333333 C 4.72,18.75 4.2222222,18.246 4.2222222,17.624999 V 9.75 H 19.777778 v 7.874999 C 19.777778,18.246 19.28,18.75 18.666667,18.75 Z"
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