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.pro

@@ -22,7 +22,6 @@ SOURCES += \
     novelwriter/gui/projdetails.py \
     novelwriter/gui/projtree.py \
     novelwriter/gui/statusbar.py \
-    novelwriter/gui/theme.py \
     novelwriter/tools/build.py \
     novelwriter/tools/projwizard.py \
     novelwriter/tools/writingstats.py \