Update WebUI img to use svg images

src/webui/www/private/preferences_content.html

@@ -386,7 +386,7 @@
         <table>
             <tr>
                 <td rowspan="2">
-                    <img alt="QBT_TR(Alternative speed limits)QBT_TR[CONTEXT=MainWindow]" src="images/slow_off.png">
+                    <img src="images/slow_off.svg" style="height: 1em;" alt="QBT_TR(Alternative speed limits)QBT_TR[CONTEXT=MainWindow]">
                 </td>
                 <td><label for="up_limit_value">QBT_TR(Upload:)QBT_TR[CONTEXT=OptionsDialog]</label></td>
                 <td><input type="number" id="up_limit_value" style="width: 4em;" min="0" />&nbsp;&nbsp;QBT_TR(KiB/s)QBT_TR[CONTEXT=OptionsDialog]</td>
@@ -404,7 +404,7 @@
         <table>
             <tr>
                 <td rowspan="2">
-                    <img alt="QBT_TR(Alternative speed limits)QBT_TR[CONTEXT=MainWindow]" src="images/slow.png">
+                    <img src="images/slow.svg" style="height: 1em;" alt="QBT_TR(Alternative speed limits)QBT_TR[CONTEXT=MainWindow]">
                 </td>
                 <td><label for="alt_up_limit_value">QBT_TR(Upload:)QBT_TR[CONTEXT=OptionsDialog]</label></td>
                 <td><input type="number" id="alt_up_limit_value" style="width: 4em;" min="0" />&nbsp;&nbsp;QBT_TR(KiB/s)QBT_TR[CONTEXT=OptionsDialog]</td>