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_key.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="svg3996"
+   viewBox="0 0 24 24"
+   height="24"
+   width="24"
+   version="1.2">
+  <metadata
+     id="metadata4002">
+    <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="defs4000" />
+  <path
+     style="stroke-width:1.42857;fill:#a2694f;fill-opacity:1"
+     id="path3994"
+     d="m 7.357143,9.142857 c 0,1.045714 0.237143,2.034286 0.641429,2.93 l -5.641429,5.641429 v 2.142857 c 0,0 1.28,2.142857 2.8571429,2.142857 H 8.071429 v -2.857143 h 2.857143 V 16.285714 H 14.5 c 3.945714,0 7.142857,-3.197143 7.142857,-7.142857 C 21.642857,5.1971429 18.445714,2 14.5,2 10.554286,2 7.357143,5.1971429 7.357143,9.142857 Z M 14.5,12 c -1.577143,0 -2.857143,-1.28 -2.857143,-2.857143 0,-1.578571 1.28,-2.86 2.857143,-2.86 1.578572,0 2.857143,1.28 2.857143,2.86 C 17.357143,10.72 16.078572,12 14.5,12 Z" />
+</svg>