Place Bisq AGPL license header at top of all Java sources

[cbeams]

All .java files should adhere to the following template:

/*
 * This file is part of Bisq.
 *
 * Bisq is free software: you can redistribute it and/or modify it
 * under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or (at
 * your option) any later version.
 *
 * Bisq is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public
 * License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with Bisq. If not, see <http://www.gnu.org/licenses/>.
 */

package bisq.example;

public class Example {
}

Note the single blank line between license header and package statement and between package and class declaration.

[cbeams]

I've added common IDEA file templates in the commits above, but there appears to be a bug in IDEA that may cause them to be ignored. As such, you may still need to set them up manually in Preferences->Editor->File and Code Templates (see the Files->[Class,Interface,Enum,AnnotationType] and Includes->File Header sections). Others' experience reports would be appreciated.

[sqrrm]

Indeed, I couldn't get IDEA to read the files, had to edit them through the settings editor for it to take effect.

[cbeams]

The bug seems to be that IDEA doesn't respect toggling from Default to Project in that settings panel. I found at least some of the time, though, that attempting to toggle would cause the settings to get picked up, even though the setting didn't ultimately persist. 🤷‍♂️