Allow specifying css style classes space separated in builder file
It would be nice if I could write <property name="css-classes">suggested-action pill</property>
in ui files instead of :
<child>
<object class="GtkButton" id="test_button">
<property name="css-classes">suggested-action
pill</property>
<property name="valign">center</property>
<property name="label" translatable="yes">Test</property>
</object>
</child>