- qml_base.ts is created as a reference point for translators
- qml_en.ts is a minimal overlay translation, providing just the needed
plural forms
- adjust the script to generate the above TS files
- fixup some leftover qsTrId() calls and the qmake file
- handle `SOURCES` recursively, rather than spelling all the paths out;
most of them were outdated and some still missing
- let the Python script skip existing translations (in case we need to
manually add plurals)
- updated qml_en.ts as a result of these changes
Stats:
```
Updating '../../ui/i18n/qml_en.ts'...
Found 1703 source text(s) (109 new and 1594 already existing)
Kept 35 obsolete entries
Same-text heuristic provided 35 translation(s)
```
Lukáš Tinkl
Patryk Osmaczko
Sale Djenic
Jonathan Rainville
Pascal Precht