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/themes/default_dark.conf

@@ -0,0 +1,28 @@
+[Main]
+name       = Default Dark Theme
+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/
+
+[Palette]
+window          =  54,  54,  54
+windowtext      = 174, 174, 174
+base            =  62,  62,  62
+alternatebase   =  67,  67,  67
+text            = 174, 174, 174
+tooltipbase     = 255, 255, 192
+tooltiptext     =  21,  21,  13
+button          =  62,  62,  62
+buttontext      = 174, 174, 174
+brighttext      = 174, 174, 174
+highlight       =  44, 152, 247
+highlightedtext = 255, 255, 255
+link            =  44, 152, 247
+linkvisited     =  44, 152, 247
+
+[GUI]
+statusnone      = 150, 152, 150
+statussaved     =  39, 135,  78
+statusunsaved   = 138,  32,  32