Replace calls of JOSM API that is now removed (since JOSM 14215)

Instead of being at "Field Papers › Scanned map ...", the entry can now be found at "Imagery › FieldPapers snapshot".

use JOSM HTTPClient + catch JsonParsingException

Push translatable strings to transifex.com when building the master branch on Travis CI

This requires the environment variable `$TRANSIFEX_TOKEN` to be set to an API token for transifex.com

In this commit the version of JOSM and the gradle-josm-plugin is also bumped to the latest version.

Update references to project location, the Gradle JOSM plugin and the JOSM version to compile against

The config file is no longer needed, that's now autogenerated by the gradle-josm-plugin.
Also no need to rename the release *.jar in Travis CI, that's also done by the plugin now.

Use `ExtendedDialog` instead of `showInputDialog`

Add Gradle build script

This adds the possibility to build this project using Gradle, while the Ant build also still works.

Update plugin to work with latest JOSM API (>= 10273)

Uses LayerManager.addLayer() instead of Main.addLayer()

Fixes https://josm.openstreetmap.de/ticket/13963

Update to support the new version of Field Papers

update to JOSM 8906

see https://josm.openstreetmap.de/ticket/12005

Clears up some ant warnings.

Fetches the id and TileJSON URL from a snapshot's metadata.
Subsequently fetches imagery constraints from the TileJSON URL.

Effectively adds support for the new version of Field Papers.

Refs fieldpapers/fieldpapers#95