fixes #264

fixes #226

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Signed-off-by: Vitaly Zaitsev <vitaly@easycoding.org>

The first distribution to have nheko in it's official repositories! 

    Right now nheko is only in the `testing` repository, but this will
    be moved to `community` after a while (after which it's available
    for installation out of the box).

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Fixes #179

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Incompatible Qt version 5.5.1

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Signed-off-by: Vitaly Zaitsev <vitaly@easycoding.org>

Allows `nix-build` for basic building and `nix-env -f . -i` for installing.

lmdbxx requires support for thread_local which is not available on that clang version.

- Improve build instructions.
- Fix fullscreen image sizing.
- Remove horizontal overscroll of RoomInfoListItem.