Fix Gradle Run task

[ebbez]

Removed the Java Gradle plugin, because:

Applying the Application plugin also implicitly applies the Java plugin. The main source set is effectively the "application".

Source: https://docs.gradle.org/current/userguide/application_plugin.html

Also set the application configuration so that gradle run starts Quelea, which it does not do at the moment on a fresh clone.

[ebbez]

The build will fail since it can not download InnoSetup.