Library rescan functionality should be more controllable

Tested on:

1) Ubuntu 11.04 (amd64)
rhythmbox 0.13.3-0ubuntu6

2) Ubuntu 11.10 (amd64)
rhythmbox 2.90.1~20110908-0ubuntu1

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1) rhythmbox 0.13.3
It has a feature with a little "noisy" behaviour.

Rhythmbox automatically rescans the library on startup, which makes a lot of small filesystem operations and takes some time if there are many files have been moved or deleted from the locations which they where imported to library from.

Anyway, with rescan functionality, Rhythmbox is better than Banshee which ignores any library changes like missing items.

2) rhythmbox 2.90.1
Library doesn't reacts on items removal automatically, though on play these removed items are removed from play queue and marked with red marks in the library.
Unfortunately, there is no "Missing files" section.

Still better than Banshee which doesn't marks anything removed when trying to play it.

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I think, the optimal solution is to to turn off the automatic rescan by default, make a "Rescan library" button and get back the "Missing Files" section.

After library rescanning triggered with a "Rescan library" button (or trying to play the missing song), the missing items should be moved to "Missing Files" section, where unneeded songs can be manually removed; on the next rescan (or trying to play the songs from "Missing Files" section), songs from "Missing Files" that have become available, should be moved back to the library, the others should remain in "Missing items").