Add support for building Qt5 modules.

This commit adds basic support for building Qt5 using the Qt5 support
documented at as requested in
issue #16 (and a dependency for #15).

Architecturally within kdesrc-build, Qt5 is handled as a special type of
module-set, in the same way that KDE project modules are special-cased
using 'kde-projects'. For Qt5, we use 'qt-projects', and reuse the
existing use-modules and ignore-modules options.

The first difference is that {use,ignore}-modules applies to Qt's git
*submodules*. We pass the combination of those to Qt's `init-repository`
script as a module-subset. Currently the user will need to enter at
least a use-modules declaration for other reasons, so we would want to
setup a sample qt5 configuration to include something appropriate.

Qt5 support also involves a dedicated source code updater (based on the
basic Git support already present) and a dedicated build system. The
source code updater handles the Git update for the qt5 "supermodule"
containing `init-repository` and then calls `init-repository` to
complete the rest of the process.

Unfortunately the existing async IPC code doesn't play well with this
but the worst that happens is that kdesrc-build will have 2 updates
running at once for a time (kdesrc-build will think all of Qt is updated
once the supermodule is updated).

The build system is actually fairly standard compared to the other

There's a lot that's still missing here, including:

* documentation,
* real support for Git submodules (an open feature request for a long
* the per-distro list of Qt build dependencies not handled by
kdesrc-build, and
* support for things like Qt's `qt5_tool`.

But, it's successfully built for me with Qt 5.12. :)
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passed #1130