Update UI to better reflect the design

* Make the Answer button look nice and laid out properly.
* Merge the time and status labels, displaying simply the text
  "Calling..." until the call becomes active and then the call time
  afterwards.
* Fix info display bar in both the call and main windows so they use a
  GtkRevealer and work a lot better.
* Add a "new-call-symbolic" icon for the "Add call" button.
* General tweaks and clean-ups

Closes #55
Closes #35

src/meson.build

@@ -76,7 +76,7 @@ calls_enum_sources = gnome.mkenums_simple('enum-types',
 calls_resources = gnome.compile_resources(
    'calls-resources',
    'calls.gresources.xml',
-   source_dir: 'ui',
+   source_dir: ['ui', '../data'],
    c_name: 'call',
 )