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/syntax/grey_dark.conf

@@ -0,0 +1,25 @@
+[Main]
+name       = Grey Dark
+author     = Veronica Berglyd Olsen
+credit     = Veronica Berglyd Olsen
+url        = https://github.com/vkbo/novelWriter
+license    = CC BY-SA 4.0
+licenseurl = https://creativecommons.org/licenses/by-sa/4.0/
+
+[Syntax]
+background     =  54,  54,  54
+text           = 200, 200, 200
+link           = 200, 200, 200
+headertext     = 225, 225, 225
+headertag      = 225, 225, 225
+emphasis       = 200, 200, 200
+straightquotes = 200, 200, 200
+doublequotes   = 200, 200, 200
+singlequotes   = 200, 200, 200
+hidden         = 150, 150, 150
+keyword        = 225, 225, 225
+value          = 200, 200, 200
+spellcheckline = 200,  46,   0
+errorline      =  46, 200,   0
+replacetag     = 225, 225, 225
+modifier       = 225, 225, 225