load image previews asynchronously

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Merge pull request #2178 from mcginty/async-media-preview

media preview fixes
Fix for fallback behavior.

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Notify on incoming messages when locked.

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Fixes #2149
Add Croatian language to list

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Closes #2167
Fixes #2026
Updated language translations.

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Bump version to 2.3.3

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Support for large media transmission.

1) All attachments are now treated as streams.  A full
   attachment is never buffered in memory (except for scaled MMS
   PDU creation in that transport).

2) Fix for some of the encrypting streams.

3) Cache thumbnails in the part database. Otherwise thumbnail
   generation takes too long for larger media.

4) Eliminate part "provider."

Remaining work:

1) Provide internal mechanism for viewing attachments.

2) Provide in-conversation item progress bars for attachment
   upload and download.

3) Provide options for limiting attachment download over mobile
   data.
upload/download UI improvements

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auto-download preference placeholders, wip
redo media preferences to be fully supported API > 9