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_light.conf

@@ -0,0 +1,25 @@
+[Main]
+name       = Grey Light
+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     = 255, 255, 255
+text           =  20,  20,  20
+link           =  20,  20,  20
+headertext     =   0,   0,   0
+headertag      =   0,   0,   0
+emphasis       =  20,  20,  20
+straightquotes =  20,  20,  20
+doublequotes   =  20,  20,  20
+singlequotes   =  20,  20,  20
+hidden         = 100, 100, 100
+keyword        =   0,   0,   0
+value          =  20,  20,  20
+spellcheckline = 200,   0,   0
+errorline      =   0, 150,   0
+replacetag     =   0,   0,   0
+modifier       =   0,   0,   0