Modifying default variables and config

Renamed variables for defaults:
- flatpak_method -> default_flatpak_method: system
- defaults.paths.prefix -> default_install_prefix: /usr/local
New variables:
- default_install_method: package

tasks/helpers/flatpak_remote.yml

@@ -0,0 +1,10 @@
+# vim: set filetype=yaml.ansible :
+---
+- name: Add flatpak remote
+  become: "{{ ext_become }}"
+  community.general.flatpak_remote:
+    enabled: "{{ remote_enabled }}"
+    flatpakrepo_url: "{{ remote_url }}"
+    method: "{{ remote_method }}"
+    name: "{{ remote_name }}"
+    state: "{{ remote_state }}"