NewCallBox+CallDisplay: UI facelift

This adds a style.css file to allow the use of custom css

Fixes https://source.puri.sm/Librem5/calls/issues/115:
- Making the phone number larger
- Moving the delete button to the bottom
- Rounding all buttons, and making the grid buttons circular
- Making the call button a bit less wide

src/calls.gresources.xml

@@ -13,6 +13,7 @@
     <file preprocess="xml-stripblanks">call-record-row.ui</file>
   </gresource>
   <gresource prefix="/sm/puri/calls/">
+    <file>style.css</file>
     <file>new-call-symbolic.svg</file>
     <file>call-arrow-incoming-symbolic.svg</file>
     <file>call-arrow-incoming-missed-symbolic.svg</file>