Materials: Add tooltips to buttons

Having only `*`, `+` & `-` as reference is not intuitive. Tooltips help
users to know what each button does.

Also marked symbols as not translatable as they appear on CrowdIn.

src/Mod/Material/Gui/ModelSelect.ui

@@ -159,7 +159,7 @@
              <string>Add to favorites</string>
             </property>
             <property name="text">
-             <string>*</string>
+             <string notr="true">*</string>
             </property>
            </widget>
           </item>