Rename manifest.json to plugin-manifest.json

Flatpak uses already manifest.json

omemo/plugin-manifest.json

@@ -0,0 +1,21 @@
+{
+    "authors": [
+        "Bahtiar `kalkin-` Gadimov <bahtiar@gadimov.de>",
+        "Daniel Gultsch <daniel@gultsch.de>",
+        "Philipp Hörist <philipp@hoerist.com>"
+    ],
+    "description": "OMEMO is an XMPP Extension Protocol (XEP) for secure multi-client end-to-end encryption based on Axolotl and PEP. You need to install some dependencies, detailed in the installation instructions for your system in the GitLab wiki.",
+    "homepage": "https://dev.gajim.org/gajim/gajim-plugins/wikis/OmemoGajimPlugin",
+    "config_dialog": true,
+    "name": "OMEMO",
+    "platforms": [
+        "linux",
+        "darwin",
+        "win32"
+    ],
+    "requirements": [
+        "gajim>=1.4.0"
+    ],
+    "short_name": "omemo",
+    "version": "2.8.14"
+}