main-window: Add the 'about' action

This will be needed by the next commit to display an "About" dialog via
the corresponding entry in the app menu that will be added.

This also adds the PACKAGE_URL, PACKAGE_VERSION and VCS_TAG
configuration data.

meson.build

@@ -27,6 +27,8 @@ project('call', 'c',
 )
 
 calls_id = 'sm.puri.Calls'
+calls_homepage = 'https://source.puri.sm/Librem5/calls'
+calls_version = meson.project_version()
 
 subdir('libgdbofono')
 subdir('src')