qt: Make askpassphrase dialog behave more sanely

Set minimum sizes appropriately, and make sure that they
are enforced. Replaces #5226.

src/qt/forms/askpassphrasedialog.ui

@@ -7,7 +7,7 @@
     <x>0</x>
     <y>0</y>
     <width>598</width>
-    <height>198</height>
+    <height>222</height>
    </rect>
   </property>
   <property name="sizePolicy">
@@ -26,8 +26,14 @@
    <string>Passphrase Dialog</string>
   </property>
   <layout class="QVBoxLayout" name="verticalLayout">
+   <property name="sizeConstraint">
+    <enum>QLayout::SetMinimumSize</enum>
+   </property>
    <item>
     <widget class="QLabel" name="warningLabel">
+     <property name="text">
+      <string notr="true">Placeholder text</string>
+     </property>
      <property name="textFormat">
       <enum>Qt::RichText</enum>
      </property>
@@ -38,6 +44,9 @@
    </item>
    <item>
     <layout class="QFormLayout" name="formLayout">
+     <property name="sizeConstraint">
+      <enum>QLayout::SetMinimumSize</enum>
+     </property>
      <property name="fieldGrowthPolicy">
       <enum>QFormLayout::AllNonFixedFieldsGrow</enum>
      </property>