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/night_owl.conf

@@ -0,0 +1,45 @@
+##
+# Theme:  Night Owl
+# Source: https://github.com/sdras/night-owl-vscode-theme
+# Credit: Sarah Drasner
+##
+# Colours:
+# Background   = 011627 :   1,  22,  39
+# Current Line = 32374d :  50,  55,  77
+# Selection    = 32374d :  50,  55,  77
+# Foreground   = d6deeb : 214, 222, 235
+# Comment      = 637777 :  99, 119, 119
+# Red          = f78c6c : 247, 140, 108
+# Orange       = ecc48d : 236, 196, 141
+# Yellow       = ffeb95 : 255, 235, 149
+# Green        = addb67 : 173, 219, 103
+# Aqua         = 7fdbca : 127, 219, 202
+# Blue         = 82aaff : 130, 170, 255
+# Purple       = c792ea : 199, 146, 234
+##
+
+[Main]
+name       = Night Owl
+author     = Veronica Berglyd Olsen (adaptation)
+credit     = Sarah Drasner (color theme)
+url        = https://github.com/sdras/night-owl-vscode-theme
+license    = MIT License
+licenseurl = https://github.com/sdras/night-owl-vscode-theme/blob/master/LICENSE
+
+[Syntax]
+background     =   1,  22,  39
+text           = 214, 222, 235
+link           = 130, 170, 255
+headertext     = 130, 170, 255
+headertag      = 130, 170, 255
+emphasis       = 236, 196, 141
+straightquotes = 247, 140, 108
+doublequotes   = 173, 219, 103
+singlequotes   = 255, 235, 149
+hidden         =  99, 119, 119
+keyword        = 247, 140, 108
+value          = 199, 146, 234
+spellcheckline = 247, 140, 108
+errorline      = 173, 219, 103
+replacetag     = 127, 219, 202
+modifier       = 236, 196, 141