xfce4-xkb-plugin rework

[Igor Slepchin]

Hey folks,

So I've finally gotten tired of xfce4-xkb-plugin losing its config 
(there are quite a few open bugs about this) and decided to take a look 
into it. This ended up in some reworking of the way plugin behaves...

The root cause of losing the layout config is X resetting keyboard 
configuration to system defaults (normally in /etc/defaults/keyboard) 
whenever a new keyboard is detected. xkb-plugin listens to keyboard 
config updates from libxklavier and quietly updates its own settings 
whenever a config change happens; these new settings are then saved to 
.rc file when the panel issues the "save" signal.

Forgetting about xkb-plugin for a moment, somebody needs to reset the 
configured keyboard settings whenever X changes them underneath. 
xkb-plugin could do that itself but it seems to make more sense to make 
xfsettingsd responsible for that since it's always there and xkb-plugin 
may or may not be used; besides, xfce4-keyboard-settings already allows 
setting up keyboard layouts and xkb-plugin effectively duplicates that.

My changes follow along these lines. Keyboard layout settings are now 
fully controlled by xfce4-keyboard-settings; xfsettingsd listens to 
system keyboard config changes (through xklavier) and resets them if 
necessary (it also takes care to only update the parts of the config it 
controls, unlike the old xkb-plugin). The "new" xkb-plugin acts mostly 
as just a keyboard layout indicator, i.e., layouts are set up through 
xfce4-keyboard-settings only (xkb-plugin still controls changing layout 
per window/app atm but it learns about the configured set from xklavier 
rather than its own config). I've also made a number of other 
fixes/improvements while at it - the commit messages are hopefully 
fairly self-explanatory so I won't duplicate them here.

I'd appreciate if you guys could take a look at 
https://github.com/igors/xfce4-settings and 
https://github.com/igors/xfce4-xkb-plugin and let me if you're ok with 
these changes or prefer any of them done differently. And please let me 
know if you'd rather have me submit the patches in a different form.

Cheers,
Igor