window-list: Refactor {Window,App}Button shared code into BaseButton

BaseButton is a new class that shares the common logic of WindowButton
and AppButton. AppButton is passed to AppContextMenu so that it can reuse
code from the now public getWindowList() method.

https://bugzilla.gnome.org/show_bug.cgi?id=737486
window-list: Option to show the window list on all monitors

A new setting "show-on-all-monitors" (false by default) is available to
show window lists on all connected monitors.
The Extension object monitors conditions that require the list of
windows to be rebuilt. The WindowList and Button classes have a new
"perMonitor" property that indicates they should handle windows on
their own monitor only.

https://bugzilla.gnome.org/show_bug.cgi?id=737486
window-list: Do not hardcode overrides schema

Classic mode uses a different overrides schema, so make sure we use the
correct setting instead of hardcoding the usual org.gnome.shell.overrides
schema.

https://bugzilla.gnome.org/show_bug.cgi?id=737486
Bump version to 3.15.3

To go along GNOME Shell 3.15.3