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/solarized_light.conf

@@ -0,0 +1,25 @@
+[Main]
+name       = Solarized Light
+author     = nullbasis
+credit     = Ethan Schoonover
+url        = https://ethanschoonover.com/solarized/
+license    = MIT
+licenseurl = https://github.com/altercation/solarized/blob/master/LICENSE
+
+[Syntax]
+background     = 253, 246, 227
+text           =   0,  43,  54
+link           =  38, 139, 210
+headertext     =  88, 110, 117
+headertag      =  42, 161, 152
+emphasis       =  38, 139, 210
+straightquotes =  211, 54, 130
+doublequotes   =  42, 161, 152
+singlequotes   =  42, 161, 152
+hidden         =  88, 110, 117
+keyword        = 133, 153,   0
+value          = 203,  75,  22
+spellcheckline = 203,  75,  22
+errorline      = 220,  50,  47
+replacetag     = 133, 153,   0
+modifier       = 181, 137,   0