sharing: Make Orca to read the "Add Folder" row in Media Sharing

This MR is meant to be backported to gnome-46, so it's not the same thing as !2498

00d9cd01 replaced the "Add Folder" a11y label by a tooltip. However, this doesn't work and Orca doesn't read the row properly; it simply says "not selected". The a11y label already didn't work prior to 00d9cd01.

To fix this, transform the tooltip into a GtkLabel. Also, make the row to have the same padding so it's consistent with the "Add Input Source..." row from Keyboard > Input Methods. I won't add an ellipsis because it will break string freeze.

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