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_plus.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="svg5808"
+   viewBox="0 0 24 24"
+   height="24"
+   width="24"
+   version="1.2">
+  <metadata
+     id="metadata5814">
+    <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="defs5812" />
+  <path
+     style="stroke-width:1.25;fill:#718c00;fill-opacity:1"
+     id="path5806"
+     d="m 19.5,9.5 h -5 v -5 C 14.5,3.12 13.38,2 12,2 10.62,2 9.5,3.12 9.5,4.5 l 0.08875,5 H 4.5 C 3.12,9.5 2,10.62 2,12 c 0,1.38 1.12,2.5 2.5,2.5 L 9.58875,14.41125 9.5,19.5 c 0,1.38 1.12,2.5 2.5,2.5 1.38,0 2.5,-1.12 2.5,-2.5 V 14.41125 L 19.5,14.5 C 20.88,14.5 22,13.38 22,12 22,10.62 20.88,9.5 19.5,9.5 Z" />
+</svg>