Merge #12616: Set modal overlay hide button as default

cfdd895 qt: Set modal overlay hide button as default (João Barbosa)

Pull request description:

  Without this change the only way to close the modal overlay is to click the hide button. Setting the button to default allows to activate it with the ENTER key.

  Before:
  <img width="849" alt="screen shot 2018-03-06 at 15 14 23" src="https://user-images.githubusercontent.com/3534524/37040276-58af9ce0-2151-11e8-8c55-50acdea669d9.png">

  After:
  <img width="848" alt="screen shot 2018-03-06 at 15 12 41" src="https://user-images.githubusercontent.com/3534524/37040294-650d1c9c-2151-11e8-8245-2da250a71b3d.png">

Tree-SHA512: a93ef440a507843ed7870fd07a693af93dd97c8fce2fb6824c69a227b5dee258f340bf1ae344da32a9dd6e6cb2330f72db9dac9635bbd34184c3e7f8476a472e

Author: laanwj

src/qt/forms/modaloverlay.ui

@@ -351,6 +351,12 @@ QLabel { color: rgb(40,40,40);  }</string>
              <property name="text">
               <string>Hide</string>
              </property>
+             <property name="focusPolicy">
+              <enum>Qt::StrongFocus</enum>
+             </property>
+             <property name="default">
+              <bool>true</bool>
+             </property>
             </widget>
            </item>
           </layout>