l2: sort and filter the forwarding table from its header

The table lists every learned and static entry in one flat block, which is
fine with a handful and unusable with a few hundred: finding out where one
MAC sits, or what a port has learned, meant reading the whole thing.

Make the column headings sort and give each one a filter box. Filters are
substring matches combined with AND, so "port 8 + static" is two keystrokes.
A counter above the table shows matched out of total, so a filter that hides
everything is obvious rather than looking like an empty table.

Sorting keeps the CPU entry from comparing as a number - it sorts last
instead of landing between ports 8 and 9, where a string-vs-number compare
would otherwise put it.

The filter inputs live inside the existing header cells rather than in a
second row: the paint loop addresses data rows as rows[i+1], and a second
header row would have shifted every one of them.

html/i18n.js

@@ -117,6 +117,7 @@ var LANG = {
     l2_col_port: 'Port',
     l2_col_type: 'Type',
     l2_col_remove: 'Remove Entry',
+    l2_shown: 'Shown:',
     l2_delete: 'Delete',
     l2_static: 'static',
     l2_learned: 'learned',
@@ -296,6 +297,7 @@ var LANG = {
     l2_col_port: 'ポート',
     l2_col_type: 'タイプ',
     l2_col_remove: 'エントリ削除',
+    l2_shown: 'Shown:',
     l2_delete: '削除',
     l2_static: '静的',
     l2_learned: '学習',
@@ -475,6 +477,7 @@ var LANG = {
     l2_col_port: '端口',
     l2_col_type: '类型',
     l2_col_remove: '删除条目',
+    l2_shown: 'Shown:',
     l2_delete: '删除',
     l2_static: '静态',
     l2_learned: '动态学习',