Add missing .gitmodules file

Issue-Ref: 3620
Signed-off-by: Alexander Diewald <diewald@fortiss.org>

.gitmodules

+[submodule "org.fortiss.af3.rcp.application/.settings"]
+	path = org.fortiss.af3.rcp.application/.settings
+	url = https://git.fortiss.org/af3/plugin-settings.git
+	branch = master