Gsettings controls keyboard layouts

Both text input and key input (virtual-keyboard) work. Layout can change at runtime. Implemented the Polish language keyboard.

Not implemented: switching layouts from within the keyboard.

Builds on top of !24 (merged)

Closes #7 (closed)

Test by adding the Polish language to the language list:

gsettings set org.gnome.desktop.input-sources sources "[('xkb', 'us'), ('xkb', 'pl')]"

and then by switching between the languages, or by changing list contents:

# gsettings set org.gnome.desktop.input-sources current 0
# gsettings set org.gnome.desktop.input-sources current 1

The pl layout will have the AltGr key displayed as ą7?, with two levels of alternative keys.

Can provide input both to IM-aware applications (GTK) and unaware ones (via virtual-keyboard).