Spaced repetition-memory cards

New features:
1. Image scaling is now supported during training for better viewing.
2. You can now copy the card text to the clipboard by long pressing.

Bug fixes:
1. When hiding a collection (pressing the eye icon next to the collection), the page no longer jumps or scrolls up.
2. Text input now properly considers whether the language is left-to-right or right-to-left.
3. Duplicates already present in the collection are now correctly detected when importing a ready-made collection.