Merge bitcoin-core/gui#871: qt, docs: Unify term "clipboard"

3bbdbc0a5e qt, docs: Unify term "clipboard" (Hennadii Stepanov)

Pull request description:

  A translator on Transifex noticed:
  > The term "system clipboard" appears twice. The term "clipboard" appears 10 times. Perhaps we could standardize on just saying "clipboard"?

  This PR addresses this issue.

ACKs for top commit:
  davidgumberg:
    ACK https://github.com/bitcoin-core/gui/pull/871/commits/3bbdbc0a5e1b409969cedaf249d1d01dea9bcf73
  pablomartin4btc:
    ACK 3bbdbc0a5e

Tree-SHA512: 61a100f60890d81122a4b8ce3e2cb7d355c7fb643de3196573f7f9107c6f52fa0b3e7a4f743ce2833e8c67b9cdad3568b761d730fef5c9781f5e1c45252888c4

src/qt/forms/addressbookpage.ui

@@ -78,7 +78,7 @@
      <item>
       <widget class="QPushButton" name="copyAddress">
        <property name="toolTip">
-        <string>Copy the currently selected address to the system clipboard</string>
+        <string>Copy the currently selected address to the clipboard</string>
        </property>
        <property name="text">
         <string>&amp;Copy</string>

src/qt/forms/signverifymessagedialog.ui

@@ -137,7 +137,7 @@
          <item>
           <widget class="QPushButton" name="copySignatureButton_SM">
            <property name="toolTip">
-            <string>Copy the current signature to the system clipboard</string>
+            <string>Copy the current signature to the clipboard</string>
            </property>
            <property name="text">
             <string/>