Simplify text width settings (#943)

* Remove the fixed text width switch, resolves #924
* Update tests
* Clean up code to set text width and add to viewer

tests/reference/baseConfig_novelwriter.conf

@@ -1,5 +1,5 @@
 [Main]
-timestamp = 2021-09-15 14:39:52
+timestamp = 2021-12-31 16:45:32
 theme = default
 syntax = default_light
 icons = typicons_light
@@ -30,7 +30,6 @@ emphlabels = True
 [Editor]
 textfont = None
 textsize = 12
-fixedwidth = True
 width = 600
 margin = 40
 tabwidth = 40