Generalize support for extra compilers

Allow for different extra compilers which may get called to generate
additional code for the code model. The build system is expected to
know what files are generated from which source file and the extra
compilers know how to generate the content of those files, without
touching the build directory. the uic adapter is refactored to be
the first such extra compiler.

The extra compiler is run when an editor for its source document
loses focus, or after a timeout of 1s when the source document has
been changed.

Change-Id: I13c110c61120c812f02639a3684144daf8979b37
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>