improv: Show regular image preview, as well as server icon preview in filebrowser upon image selection

# Conflicts:
#   serverpackcreator-gui/src/main/kotlin/de/griefed/serverpackcreator/gui/window/configs/filebrowser/controller/FileSelectionListener.kt
#   serverpackcreator-gui/src/main/kotlin/de/griefed/serverpackcreator/gui/window/configs/filebrowser/controller/TreeExpandListener.kt
#   serverpackcreator-gui/src/main/kotlin/de/griefed/serverpackcreator/gui/window/configs/filebrowser/model/FileBrowserModel.kt
#   serverpackcreator-gui/src/main/kotlin/de/griefed/serverpackcreator/gui/window/configs/filebrowser/view/TableScrollPane.kt
#   serverpackcreator-gui/src/main/kotlin/de/griefed/serverpackcreator/gui/window/configs/filebrowser/view/TreeScrollPane.kt

Several improvements

See merge request !437