xfce4-settings-mime UI/UX enhancements

xfce4-settings-mime (Default Applications) has a somewhat limited interface when it comes to managing multiple entries in the Others tab.

Besides #346 (closed) there are more small UI/UX improvements that could be implemented there:

1. The column order is not the best. The Default Application should be the second column (before the Status since it's more important and also more related to the first column.

2. The search filter currently matches MIME types only. It could also match (search through) Default Application values so the user is more easily able to find MIME types of a certain app.

3. Each change currently forces the list to reorder immediately so when the list is sorted by a default app, rows jump up and down. It would be more appropriate here if "lazy" table refresh was used. User could then manually trigger the refresh by switching one or the other tabs and if that's not sufficient, a small refresh button could also easily be added (while I don't believe it is necessary, it's a standard UI trick).

Since these are smaller related enhancements, I reported them under a single issue rather than to produce three. Not a problem to split if that's desired. On the other hand, this could also be joined with #346 (closed).