Support for Java 12

[daniele-athome]

Try to make Kontalk compatible with Java 12 (one example of bug is #112).

[cosminribo]

Support for a more recent JVM would also help solve some high DPI issues.
With java-8, the export GDK_SCALE=2 option is not respected by the client app. With java-11, the Kontakt client looks good, upscaled by a factor of 2 when the export GKD_SCALE has been issued, but does not connect to the network.