UI: Adjust a few margins

-Remove 9px margins around central widget. This saves 18px of vertical
space. This will be helpful when the source context menu is added,
which is 24px tall.

-Adjusts margins in transitions dock. The top of the transitions dock
wasn't lined up with the top of the controls dock, which made it look
bad.

UI/forms/OBSBasic.ui

@@ -38,6 +38,21 @@
   </property>
   <widget class="QWidget" name="centralwidget">
    <layout class="QVBoxLayout" name="verticalLayout">
+    <property name="spacing">
+     <number>0</number>
+    </property>
+    <property name="leftMargin">
+     <number>0</number>
+    </property>
+    <property name="topMargin">
+     <number>0</number>
+    </property>
+    <property name="rightMargin">
+     <number>0</number>
+    </property>
+    <property name="bottomMargin">
+     <number>0</number>
+    </property>
     <item>
      <layout class="QHBoxLayout" name="horizontalLayout_2">
       <item>
@@ -853,19 +868,19 @@
       <widget class="QWidget" name="transitionsContainer" native="true">
        <layout class="QVBoxLayout" name="verticalLayout_2">
         <property name="spacing">
-         <number>4</number>
+         <number>2</number>
         </property>
         <property name="leftMargin">
-         <number>1</number>
+         <number>0</number>
         </property>
         <property name="topMargin">
-         <number>1</number>
+         <number>0</number>
         </property>
         <property name="rightMargin">
-         <number>1</number>
+         <number>0</number>
         </property>
         <property name="bottomMargin">
-         <number>2</number>
+         <number>0</number>
         </property>
         <item>
          <widget class="QComboBox" name="transitions">