gui: Move static placeholder texts to forms

There was an issue around the time of Qt 4.6 when placeholder text was
introduced, that caused a compile failure when it was specified in the
form.

As a workaround the placeholder texts were moved to the code.

Qt 4 hasn't been relevant to us for ages. So move all (non-parametrized)
placeholder texts to the form files instead.

It's better to keep this kind of text content together. Makes sure
translate/no-translate status is kept as it is.

src/qt/forms/openuridialog.ui

@@ -24,7 +24,11 @@
       </widget>
      </item>
      <item>
-      <widget class="QValidatedLineEdit" name="uriEdit"/>
+      <widget class="QValidatedLineEdit" name="uriEdit">
+       <property name="placeholderText">
+        <string notr="true">bitcoin:</string>
+       </property>
+      </widget>
      </item>
     </layout>
    </item>

src/qt/forms/optionsdialog.ui

@@ -685,6 +685,9 @@
            <property name="toolTip">
             <string>Third party URLs (e.g. a block explorer) that appear in the transactions tab as context menu items. %s in the URL is replaced by transaction hash. Multiple URLs are separated by vertical bar |.</string>
            </property>
+           <property name="placeholderText">
+            <string notr="true">https://example.com/tx/%s</string>
+           </property>
           </widget>
          </item>
         </layout>

src/qt/forms/sendcoinsentry.ui

@@ -144,6 +144,9 @@
       <property name="toolTip">
        <string>Enter a label for this address to add it to the list of used addresses</string>
       </property>
+      <property name="placeholderText">
+       <string>Enter a label for this address to add it to your address book</string>
+      </property>
      </widget>
     </item>
     <item row="2" column="0">

src/qt/forms/signverifymessagedialog.ui

@@ -121,6 +121,9 @@
          </property>
          <item>
           <widget class="QLineEdit" name="signatureOut_SM">
+           <property name="placeholderText">
+            <string>Click "Sign Message" to generate signature</string>
+           </property>
            <property name="font">
             <font>
              <italic>true</italic>