Fix build system to properly update translation source files

[matty0ung]

This is a step towards fixing https://github.com/Brewtarget/brewtarget/issues/670

I'm doing two things here:

• Discarding the ".po" translation source files and deciding that the ".ts" files are the master translation files. (The ".ts" files are the Qt native format. We only needed the ".po" ones when we were using Launchpad.
• Updating the build system so that it automatically adds new entries to the ".ts" files when new translatable strings get added to the .cpp or .ui files. (Essentially this is equivalent to running lupdate on all the ".ts" files when we do make

Of course this is all the easy bit and actually getting the translations done is a whole other issue, but it's a step in the right direction I hope!

[matty0ung]

There's another fix to come here BTW. Just realised we're only getting the strings from cpp files and not the ui ones. But at least I know why! :smile: