Gui: Clarify cache directory as read-only in prefs, add Browse tooltip

Fixes #13608.

src/Gui/PreferencePages/DlgSettingsCacheDirectory.ui

@@ -25,7 +25,7 @@
         <item row="0" column="0">
          <widget class="QLabel" name="label">
           <property name="text">
-           <string>Location:</string>
+           <string>Location (read-only):</string>
           </property>
          </widget>
         </item>
@@ -38,6 +38,9 @@
         </item>
         <item row="0" column="3">
          <widget class="QPushButton" name="openButton">
+          <property name="toolTip">
+           <string>Browse cache directory</string>
+          </property>
           <property name="sizePolicy">
            <sizepolicy hsizetype="Fixed" vsizetype="Fixed">
             <horstretch>0</horstretch>