Make category filtering a reusable function for live filtering

Can now be used anywhere where a "real-time" filter is desirable.
As a usage example, it's implemented for the list of installable
plugins. Should work anywhere, though – needs an input element,
a list of target elements and an element within those target
elements to contain the search term.

Also made the function for the reset button for this reusable.

References #154

templates/2k11/admin/style.css

@@ -507,7 +507,8 @@ form > button,
 }
 
 .overviewListForm > button,
-.ping_services .form_check {
+.ping_services .form_check,
+.plugins_installable {
     margin-top: 0;
     margin-bottom: 0;
 }
@@ -2708,12 +2709,18 @@ img.mfp-img {
     .entryproperties_access_groups,
     .entryproperties_access_users,
     .entryproperties_access_author,
-    .entryproperties_markup {
+    .entryproperties_markup,
+    #plugin_groups,
+    #plugin_filter {
         float: left;
         margin-right: 2%;
         width: 48%;
     }
 
+    #plugin_groups .form_buttons {
+        margin-bottom: 0;
+    }
+
     .entryproperties_customfields {
         clear: both;
     }