Add GTK+ popup window to indicate volume changes

This commit also adds appropriate options to the preferences page
to switch between a native GTK+ popup window and libnotify to indicate
volume changes. Note that this patch reverts commit 8f072e3 which added
a command-line switch to disable notifications. This is now possible
through the preferences page.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Use *-panel variant of default icons (closes #23)

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Remove preprocessor check for DATADIR

The DATADIR symbol is always defined by autotools so checking if it is
defined in volumeicon.c is futile. Besides, defining it as "../data"
assumes that if volumeicon is run locally, it is run from the `src'
directory.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Destroy preferences window during shutdown

Changes in the preferences dialog are only saved to the config when the
window is destroyed. This commit makes sure the preferences window is
explicitly destroyed during shutdown of volumeicon to store any
outstanding changes.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Merge pull request #26 from nkoep/popup-window-notification-rebase

Add GTK+ popup window to indicate volume changes