precise layer selection option (#3223)

Author: DelazJ

src/ui/qgssnappingdialogbase.ui

@@ -38,7 +38,7 @@
        </item>
        <item>
         <property name="text">
-         <string>All layers</string>
+         <string>All visible layers</string>
         </property>
        </item>
        <item>