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

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