META: Better translation management API in locale module

This is a meta issue for different improvements in locale module's translation API on the administration side. Look into sub-issues for more details.

Sub-issues

#1188430: Rip out textgroup support from locale module
#1223502: Update for ripping out textgroup support in Drupal locale module
#1191662: Migrate away from textgroups to own storage in i18n

#361597: CRUD API for locale source and locale target strings

#1219236: Locale module includes 1350+ lines of unneeded code on all page loads
#1189184: OOP & PSR-0-ify gettext .po file parsing and generation

Parent issue

#1260534: META-META: Make language support awesome in Drupal 8