Add new key for ubuntu repo that causes travis to fail.

Link mtxclient dynamically and update to Boost 1.69 for CI build

Simplify linkifyMessage.  Fixes #18.  Thanks @MonokelPinguin 

Parsing html as xml has inherent problems, most notable there are many
matrix clients that don't escape ampersands in urls of mx-replies, etc.
(See issue #18)

This also removes the replacement of <mx-reply> as it isn't strictly
needed.

Also the QRegExp is replaced with the Qt5 QRegularExpression for
perfomance and because it supports lookahead and lookbehind.

I'm pretty sure that the original code also replaced href="" with
href=\"\", which was probably wrong, but I'm not to sure about that.

Fixes #18

improve shell scripts and update .gitignore.  Thanks @zapashcanon 

Remove LICENSE file in favor of COPYING

Get rid of implicit conversion in text input height calculation.  Thanks @vtronko 

Don't let multiline text background overlap top border of text widget.  Fixes #39 
Thanks @vtronko 

Apparently the nlohmann library is not available in the standard
repositories.  I would rather build it than add universe deps.

Update JSON build dependencies, change the dockerfile and other
CI files that relate to this.  Also update target mtxclient
revision.

Updated Dutch translation (thanks @Vistaus )