Use KConfigXT in KWin::Options

For the time being the current design of Options is more or less kept to
not have to adjust KWin code all over the place. Also for some parts the
generated class from KConfigXT cannot be used due to inter-settings
dependencies defined in the setters.

Options now holds a pointer to a Settings object which is generated from
KConfigXT and uses it to read the default values and the individual
settings. This means the static default value methods are dropped and the
variables are initialized with a normal default value (all int 0, all
boolean false and so on) in the initializer list. Afterwards the values
are set to the correct default value through KConfigXT.

So far for the first step only Windows category is using KConfigXT.

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