Add more consistency to docking panels and widgets

* Dock panels shouldn't have any frame border or anything like that.
* They should have an external margin of 3 pixels at their border,
  but then no further margins on the layouts (like sub-controls for
  the pipeline state views.
* Toolbars should be Raised & Panel frames. Later we'll replace them
  with actual QToolBars to better customise the painting.

qrenderdoc/Windows/PythonShell.ui

@@ -18,16 +18,16 @@
     <number>0</number>
    </property>
    <property name="leftMargin">
-    <number>2</number>
+    <number>3</number>
    </property>
    <property name="topMargin">
-    <number>2</number>
+    <number>3</number>
    </property>
    <property name="rightMargin">
-    <number>2</number>
+    <number>3</number>
    </property>
    <property name="bottomMargin">
-    <number>2</number>
+    <number>3</number>
    </property>
    <item>
     <widget class="QTabWidget" name="tabWidget">