Fontlist: Move cache file in a subdirectory, so as not to mess with

Cache's state tracking

Re #7510

Legacy cache has to be deleted in Cache,
*before* the first getDiskCache call.

Reword the "Clear caches" Dev menu entry & callback:
We now have an actual "Restart" action (on most, if not all, platforms).
And we clear the whole cache folder, which may host other things (e.g.,
fontlist)

frontend/cache.lua

@@ -39,6 +39,9 @@ end
 
 local cache_path = DataStorage:getDataDir() .. "/cache/"
 
+-- NOTE: Before 2021.04, fontlist used to squat our folder, needlessly polluting our state tracking.
+os.remove(cache_path .. "/fontinfo.dat")
+
 --[[
 -- return a snapshot of disk cached items for subsequent check
 --]]