use bottom_margin etc instead of CSS padding

this fixes #41 and is a GTK issue

Author: vixalien

data/stylus/style.styl

@@ -11,7 +11,6 @@
 
 .text-view {
   background-color: transparent;
-  padding: 12px;
   color: inherit;
 }
 

data/ui/window.ui

@@ -45,6 +45,10 @@
               <object class="GtkTextView" id="text">
                 <property name="editable">true</property>
                 <property name="wrap-mode">word-char</property>
+                <property name="top-margin">12</property>
+                <property name="bottom-margin">12</property>
+                <property name="left-margin">12</property>
+                <property name="right-margin">12</property>
                 <style>
                   <class name="text-view"/>
                 </style>