basesrc: Only force-update the duration for dynamic sources when doing the DURATION query

Doing it after every single create() is not very efficient and not necessary.
Especially on network file systems fstat() is not cached and causes network
traffic, making the source possibly unusable slow.

https://bugzilla.gnome.org/show_bug.cgi?id=652037
input-selector: send notify::active signal for input-selector pads.

https://bugzilla.gnome.org/show_bug.cgi?id=701319