Proposal of translation support

Initial proposal of translation support:
* Theme-specific translations - theme creator makes directory 'translations'(he can specify its name in theme config), and puts compiled(via lrelease) *.qm files/
* There is a special component - 'TextConstants' which is basically pool of pretranslated strings, so theme creator can use them, and have localization out of the box, without providing custom .qm files.
* All qm files should be named as xx_YY.qml of xx.qml, where xx - language code, YY - country code
* There is build support for Components translation files in data/translations and theme-specific translations(which are not used, since standard themes use TextConstants)(so we probably don't need that)