wayland: write wl_data_offer data asynchronously

Currently writing wl_data_offer data into the fd is 1) synchronous, which
is noticeable when transferring large amounts of data, and 2) buggy, write()
error checking is done on the accumulator, breaking both the written data
accounting and error checking itself.

Fix both by making writes asynchonous through GOutputStream, the operation
is spun off and either finished, or cancelled if new data is stored in the
selection while the transfer is active.
gtkplacessidebar: Don't change location if clicked on eject button

Currently we change the current location if we click the eject button of
a mount.

Check whether the user actually clicked the eject button and don't
change location in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=737983
gtkplacessidebar: Open $HOME after unmounting or ejecting

When a drive is ejected or a volume unmounted the current directory
doesn't change most of the times being empty or being a directory that
user shouldn't take care about, like /run/media

Seems more useful to change to $HOME directory in that case so the user
can see something useful and familiar just after unmounting.

https://bugzilla.gnome.org/show_bug.cgi?id=737983