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_minus.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"
+   version="1.2"
+   width="24"
+   height="24"
+   viewBox="0 0 24 24"
+   id="svg5204">
+  <metadata
+     id="metadata5210">
+    <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="defs5208" />
+  <path
+     d="M 19.5,9.5 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 h 15 C 20.88,14.5 22,13.38 22,12 22,10.62 20.88,9.5 19.5,9.5 Z"
+     id="path5202"
+     style="stroke-width:1.25;fill:#d64848;fill-opacity:1" />
+</svg>