trayicon: set XAyatanaLabel attribute for "Show next break time in tray icon"

[deltragon]

Fixes #643

This a non-standard attribute that is not supported by many desktops, e.g. KDE and snixembed.
However, this also did not work with libappindicator on KDE before.

This should fix the regression in #643 for all desktops that support both StatusNotifierItem and the non-standard XAyatanaLabel property, notably GNOME with the AppIndicator and KStatusNotifierItem Support extension.

[deltragon]

I have not tested this on GNOME yet, only on KDE (where it continues to not show the time in the tray icon).
I will try and test this on GNOME the next few days.

[AdamPS]

Reviewed and tested on Gnome

[archisman-panigrahi]

It shows up prominently in GNOME

Shows up as a tooltip in KDE6 (no change before and after this PR)

Also works in XFCE.

However, it does not work with snixembed in LXDE

[archisman-panigrahi]

Does not work with snixembed in MATE

[archisman-panigrahi]

We have to make some decision on whether we want to support this feature on snixembed. Otherwise, I approve this PR.

[archisman-panigrahi]

Given it works in some form in GNOME, KDE, XFCE and Cinnamon (image attached), I am in favor of merging this.

@deltragon If you think this can be somehow made to work with snixembed please let us know, otherwise, I will just merge it as it is.

Thank you all!

[deltragon]

Thanks!
The fact that it doesn't change the tooltip at all with snixembed is surprising - in theory, it does have support for the tooltip (and also shows the original tooltip). So this might just be an issue of it not realising the tooltip was updated.
I can look into that.