Added some "What's This" information

Added some texts to the "What's This" information of some buttons or other
widgets that lacked them. This will help the user to discover the provided
features.

Specifically, help is added for the filter fields and the remote dock.
See discussions in issues #1311 and #1312.

src/RemoteDock.ui

@@ -74,7 +74,11 @@
     </layout>
    </item>
    <item>
-    <widget class="QTreeView" name="treeStructure"/>
+    <widget class="QTreeView" name="treeStructure">
+     <property name="whatsThis">
+      <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;In this pane, remote databases from dbhub.io website can be added to DB4S. First you need an identity:&lt;/p&gt;&lt;ol style=&quot;margin-top: 0px; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; -qt-list-indent: 1;&quot;&gt;&lt;li style=&quot; margin-top:12px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;&quot;&gt;Login to the dbhub.io website (use your GitHub credentials or whatever you want)&lt;/li&gt;&lt;li style=&quot; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;&quot;&gt;Click the button to create a DB4S certificate (that's your identity). That'll give you a certificate file (save it to your local disk).&lt;/li&gt;&lt;li style=&quot; margin-top:0px; margin-bottom:12px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;&quot;&gt;Go to the Remote tab in DB4S Preferences. Click the button to add a new certificate to DB4S and choose the just downloaded certificate file.&lt;/li&gt;&lt;/ol&gt;&lt;p&gt;Now the Remote panel shows your identity and you can add remote databases.&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
+     </property>
+    </widget>
    </item>
   </layout>
  </widget>