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_light/typ_cancel.svg

@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   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"
+   id="svg934"
+   viewBox="0 0 24 24"
+   height="24"
+   width="24"
+   version="1.2">
+  <metadata
+     id="metadata940">
+    <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>
+  <defs
+     id="defs938" />
+  <path
+     style="stroke-width:1.25;fill:#c82829;fill-opacity:1"
+     id="path932"
+     d="M 12,2 C 6.48625,2 2,6.48625 2,12 2,17.51375 6.48625,22 12,22 17.51375,22 22,17.51375 22,12 22,6.48625 17.51375,2 12,2 Z M 5.75,12 c 0,-1.04 0.28,-2.005 0.73,-2.86875 L 14.86875,17.52 C 14.005,17.97 13.04,18.25 12,18.25 8.55375,18.25 5.75,15.44625 5.75,12 Z M 17.52,14.86875 9.13125,6.48 C 9.995,6.03 10.96,5.75 12,5.75 c 3.44625,0 6.25,2.80375 6.25,6.25 0,1.04 -0.28,2.005 -0.73,2.86875 z" />
+</svg>