Merge bitcoin-core/gui#217: qt: Make warning label look clickable

67c59ae qt: Make warning label look clickable (Jarol Rodriguez)

Pull request description:

  The warning icon on the overview page indicates that there is something important the user should know about, but a user may not be aware that they can click it because, on `master`, the warning label does not look clickable. As detailed in issue bitcoin#23, the reason to make it look clickable is that it if they "had a more clickable-appearance (borders or beveled button edges) it could help users more quickly understand what they are being alerted to."

  This PR removes the `flat` property from both `QPushButton`'s to make them look like a button, and therefore clickable. Furthermore, it updates the `Maximum Width` to `45` to fix the small hit-box issue outlined in issue bitcoin#215.

  Below are screenshots showing how the warning icon looks under `master` and this `PR`:

  **macOS 11.1: Qt 5.15**
  | Master        | PR               |
  | ----------- | ----------- |
  |  <img width="754" alt="Screen Shot 2021-02-22 at 5 00 40 PM" src="https://user-images.githubusercontent.com/23396902/108776135-f6d50380-752f-11eb-9f96-25163c6a2a02.png"> | <img width="754" alt="Screen Shot 2021-02-22 at 3 08 40 PM" src="https://user-images.githubusercontent.com/23396902/108776068-e0c74300-752f-11eb-9545-3580e2b8f187.png"> |

  **Ubuntu 20.04: Qt 5.12**

  | Master        | PR               |
  | ----------- | ----------- |
  | <img width="783" alt="Screen Shot 2021-02-22 at 4 57 32 PM" src="https://user-images.githubusercontent.com/23396902/108776249-284dcf00-7530-11eb-8325-7fe13a9243a7.png"> |   ![Screen Shot 2021-02-22 at 4 12 54 PM](https://user-images.githubusercontent.com/23396902/108776428-60eda880-7530-11eb-8999-59ddd70de85f.png) |

  Closes bitcoin#23
  Closes bitcoin#215

ACKs for top commit:
  Talkless:
    tACK 67c59ae, tested on Debian Sid. Does look as expected.

Tree-SHA512: 2b7302fb990ea49e2f01df6f4a23e2bc3de0797da89deaeb299742e6b285a0c21ea80d8259dc0222640cccc2bccc4ea09df443b9a11bf8b88a828e5fb2aec12c

src/qt/forms/overviewpage.ui

@@ -68,7 +68,7 @@
               </property>
               <property name="maximumSize">
                <size>
-                <width>30</width>
+                <width>45</width>
                 <height>16777215</height>
                </size>
               </property>
@@ -89,9 +89,6 @@
                 <height>24</height>
                </size>
               </property>
-              <property name="flat">
-               <bool>true</bool>
-              </property>
              </widget>
             </item>
             <item>
@@ -406,7 +403,7 @@
               </property>
               <property name="maximumSize">
                <size>
-                <width>30</width>
+                <width>45</width>
                 <height>16777215</height>
                </size>
               </property>
@@ -427,9 +424,6 @@
                 <height>24</height>
                </size>
               </property>
-              <property name="flat">
-               <bool>true</bool>
-              </property>
              </widget>
             </item>
             <item>