Drop grey icon themes (#869)

* Add settings translator for icon themes
* Rename `typicons_colour_*` to `typicons_*`
* Delete grey icon themes
* Delete outdated paragraph in main README
* Fix tests

tests/reference/baseConfig_novelwriter.conf

@@ -1,8 +1,8 @@
 [Main]
-timestamp = 2021-08-17 20:59:39
+timestamp = 2021-08-26 17:48:02
 theme = default
 syntax = default_light
-icons = typicons_colour_light
+icons = typicons_light
 guidark = False
 guifont = 
 guifontsize = 11

tests/reference/guiPreferences_novelwriter.conf

@@ -1,8 +1,8 @@
 [Main]
-timestamp = 2021-08-17 20:59:42
+timestamp = 2021-08-26 17:48:04
 theme = default
 syntax = default_light
-icons = typicons_colour_light
+icons = typicons_light
 guidark = True
 guifont = Sans
 guifontsize = 12
@@ -13,7 +13,7 @@ hidehscroll = True
 
 [Sizes]
 geometry = 1200, 650
-preferences = 700, 615
+preferences = 670, 589
 treecols = 200, 50, 30
 novelcols = 200, 50
 projcols = 200, 60, 140