Add a UPDATE_TRANSLATIONS option

This commit adds a UPDATE_TRANSLATIONS to cmake that can be used to regenerate (aka lupdate) cockatrice's translation files.
When cmake is first run and checks for the Qt libraries, it will search for lupdate/release.
When cmake is run with this option:
```
cmake .. -DUPDATE_TRANSLATIONS=1
```
it will update the translation files.
Please note that there's a known problematic feature (let's call it a bug): when the UPDATE_TRANSLATIONS option is enabled, the translation files are treated as targets, so a "make clean" will happily delete them. So, you  probably want to enable this option only to update translations and disable it soon after.

References issue #287 and pr #291