fixes #313

fixes #438

fixes #283

fixes #393

fixes #357
fixes #335
fixes #230

Add method to convert matrix content URIs to http links

fixes #40

On macOS the modal has some extra space around the main content
that might make it unusable with a big enough image.

* Refactor widget items to use same interface

* Support audio, video, generic file for pasting

* Add utils function for human readable file sizes

* Set correct MIME type for media messages

This change also determines the size of the upload once from the
ContentLengthHeader, rather than seeking the QIODevice and asking for
its size. This prevents any future trouble in case the QIODevice is
sequential (cannot be seeked). The MIME type is also determined at
upload once, rather than using the QIODevice and the underlying data
inside.

* Allow for file urls to be used as fall-back

This fixes an issue on macOS which uses `text/uri-list` for copying
files to the clipboard.

fixes #228