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/tomorrow_night_bright.conf

@@ -0,0 +1,45 @@
+##
+# Theme:  Tomorrow Night Bright
+# Source: https://github.com/chriskempson/tomorrow-theme
+# Credit: Chris Kempson
+##
+# Colours:
+# Background   = 000000 :   0,   0,   0
+# Current Line = 2a2a2a :  42,  42,  42
+# Selection    = 424242 :  66,  66,  66
+# Foreground   = eaeaea : 234, 234, 234
+# Comment      = 969896 : 150, 152, 150
+# Red          = d54e53 : 213,  78,  83
+# Orange       = e78c45 : 231, 140,  69
+# Yellow       = e7c547 : 231, 197,  71
+# Green        = b9ca4a : 185, 202,  74
+# Aqua         = 70c0b1 : 112, 192, 177
+# Blue         = 7aa6da : 122, 166, 218
+# Purple       = c397d8 : 195, 151, 216
+##
+
+[Main]
+name       = Tomorrow Night Bright
+author     = Veronica Berglyd Olsen (adaptation)
+credit     = Chris Kempson (color theme)
+url        = https://github.com/chriskempson/tomorrow-theme
+license    = MIT License
+licenseurl = https://github.com/chriskempson/tomorrow-theme/blob/master/LICENSE.md
+
+[Syntax]
+background     =   0,   0,   0
+text           = 234, 234, 234
+link           = 122, 166, 218
+headertext     = 122, 166, 218
+headertag      = 122, 166, 218
+emphasis       = 231, 140,  69
+straightquotes = 213,  78,  83
+doublequotes   = 185, 202,  74
+singlequotes   = 231, 197,  71
+hidden         = 150, 152, 150
+keyword        = 213,  78,  83
+value          = 195, 151, 216
+spellcheckline = 213,  78,  83
+errorline      = 185, 202,  74
+replacetag     = 112, 192, 177
+modifier       = 231, 140,  69