datetime: Use the currently preferred way for setting accessible names

For entries that have labels associated, use the mnemonic relationship; and for
those that don't, set up the accessible name.

https://bugzilla.gnome.org/show_bug.cgi?id=708219
datetime: Sync AM/PM button's accessible name with the text displayed

Instead of hardcoding the accessible name as "Switch between AM and
PM.", set it to "AM" or "PM" depending on which label the stack switcher
is currently showing.

https://bugzilla.gnome.org/show_bug.cgi?id=708219
power: Don't show the estimated battery capacity

Since we don't currently have the data to present a good estimation.

https://bugzilla.gnome.org/show_bug.cgi?id=691187
power: Fix getting the keyboard brightness value

https://bugzilla.gnome.org/show_bug.cgi?id=706040
sharing: prevent incorrect sensitivity of the screen sharing password entry

Both the GSettings binding and "require password" switch were attempting to
control the sensitivity of the screen sharing password entry, so allow the
require password switch to control the sensitivity of the parent container
instead.

https://bugzilla.gnome.org/show_bug.cgi?id=703521