Add a sniffing rule for XML.

* Set to a weak priority, to let more specific rules be used (svg,
xhtml, etc.)
TabManager: better delay detection for closing the menu.

As suggested by stippi, we can use system_time and the CurrentMessage
when field to determine the relative time of the actual mouse clicks,
rather than the time of message delivery.
Remove useless include.
MediaRoster: remove another message to app stdout.
Move IconMenuItem.h to headers/private/tracker.

* This can be useful outside of Tracker, so make it more visible.
WebPositive: add a bookmark bar.

* This is a BMenuBar with IconMenuItems.
* Uses node monitoring to live-update the bar as bookmarks are modified.
* Bookmark folders are BNavMenus and can be browsed.
* This needs some improvements, for example handling more bookmarks than
the window width allows us to show (with an extra menu like for tabs?)

Fixes #10232.