variable_watch: Don't share memory for callbacks

The command itself is owned by the cmMakefile class, but the
cmVariableWatch which holds a pointer to the cmVariableWatchCommand via
the client_data for the callback outlives the cmMakefile class in the Qt
GUI. This means that when the cmMakefile is destroyed, the variable
watch is still in effect, but with a stale pointer.

To fix this, each callback is now a separate entity completely and
doesn't rely on the command which spawned it at all.

An example CMakeLists.txt which demonstrates the issue (only displayed
in cmake-gui, so no tests can be written for it):

    set(var 0)
    variable_watch(var)

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