fix(theme): prevent panel element headings from being clipped

Three QSS issues caused headings to render with only the top ~60%
visible:

- QGroupBox: margin-top 12px was insufficient for the title rendered
  in subcontrol-origin: margin. Increased to 16px and added 2px
  vertical padding to the title.
- QDockWidget::title: min-height 18px conflicted with padding 8px 6px,
  constraining the content area. Removed min-height to let Qt auto-size
  from padding + font metrics.
- QSint--ActionGroup QToolButton: min-height 18px forced a height that
  was then clipped by the C++ setFixedHeight(headerSize) calculation.
  Set min-height to 0px so the C++ layout controls sizing.

Closes #325

src/Gui/Stylesheets/KindredCreate.qss

@@ -251,7 +251,6 @@ QDockWidget::title {
     text-align: left;
     padding: 8px 6px;
     border-bottom: 1px solid #313244;
-    min-height: 18px;
 }
 
 QDockWidget::close-button,
@@ -733,7 +732,7 @@ QGroupBox {
     background-color: #1e1e2e;
     border: 1px solid #45475a;
     border-radius: 6px;
-    margin-top: 12px;
+    margin-top: 16px;
     padding-top: 8px;
 }
 
@@ -741,7 +740,7 @@ QGroupBox::title {
     subcontrol-origin: margin;
     subcontrol-position: top left;
     left: 12px;
-    padding: 0 4px;
+    padding: 2px 4px;
     color: #bac2de;
     background-color: #1e1e2e;
 }
@@ -1234,7 +1233,7 @@ QSint--ActionGroup QToolButton {
     border: none;
     border-radius: 4px;
     padding: 8px 6px;
-    min-height: 18px;
+    min-height: 0px;
 }
 
 QSint--ActionGroup QToolButton:hover {