* 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 

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

fixes #132 

fixes #128

fixes #25

The color scheme of nheko obeys the default color theme of Qt
(i.e. the system theme). It uses a Qt stylesheet to accomplish this,
which means replacing the color theme with a custom theme would only
be a matter of writing a new style sheet and loading it into the app.

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

Minor translation fixes

- Add icon set on mac for future packaging

* Validate both inferred and explicitly entered server addresses by attempting to call the /versions endpoint
* If the domain from the mxid fails validation, try prefixing it with 'matrix'
* Only show server address field if address validation ultimately fails

Currently translated at 100.0% (34 of 34 strings)

Translation: Nheko/Nheko
Translate-URL: https://translate.nordgedanken.de/projects/nheko/nheko/de/

Currently translated at 97.0% (33 of 34 strings)

Translation: Nheko/Nheko
Translate-URL: https://translate.nordgedanken.de/projects/nheko/nheko/nl/

Thanks @ha:matrix.org for the initial script