Use perl instead of grep to scan POTFILES.ignore and POTFILES.skip

No point in shelling out of perl to do what perl does best; besides, grep
has environment settings that can break our parsing of the result.
Add *.cpp to the search pattern for files to translate.

Exclude the files in src/gnc and src/optional/gtkmm; they shouldn't have
any user-visible strings.
Remove a bunch of no-longer-existing files from POTFILES.skip.
POTFILES.in resulting from previous make-gnucash-potfiles changes.