plugin app manager: first draft:

/!\ new optional dependency: pyyaml

this plugin manage the life cycle of external applications. Application handlers register
to it.

Data needed to launch an application as set in YAML files. Local data types are used to
get values directly from SàT:

- !sat_conf to retrieve a configuration value
- !sat_generate_pwd to generate a password
- !sat_param for parameters specified a launch

Data can be exposed when an instance is launched, this can be used to specify the port
(notably the one used for web), or a generated password.

setup.py

@@ -38,7 +38,7 @@ install_requires = [
     'cryptography',
     'pygments',
     'pygobject',
-    'PyOpenSSL',
+    'pyopenssl',
     'python-dateutil',
     'python-potr',
     'pyxdg',
@@ -51,6 +51,7 @@ install_requires = [
     'wokkel >= 0.7.1',
     'omemo >= 0.11.0',
     'omemo-backend-signal',
+    'pyyaml',
 ]
 
 extras_require = {