Merged "Preview Card" feature from Jolta Tech into the current 2.1-dev branch

Squashed the following commits:

commit 8c0b969b151993fbc328f3a0abe208dc156b4940
    Fixed the bug where preview button is shown from Add Note. Also added @JavascriptInterface decorator for Android 4.2+ compatibility
commit 56e378bc7b377ded1f6ec8b3b0c1c223b193f645
    Add missing code from JoltaTech
commit 0f611ce23fa6faa87ca866b55322168fc80c1810
    Extracted work from JoltaTech to add preview button to CardEditor.java and create PreviewClass.java, and manually brought it into the latest 2.1-dev branch.