Implement UserphraseSet
Remove unused code
Add TC for operator[] in UserphraseSet
Use UserphraseSet to avoid duplicated phrase

Closed #22
Set mode to ExistingFile in ImportDialog
Implement Export
Generate ts files for i18n
Update CMakeFiles.txt for ts generation
Update i18n related code in CMakeLists.txt
Add zh_TW translation
Load translation based on locale

Closed #11