Change doxygen tags to markdown + enhancements

Renamed Documentation.h files to README.md.
Reorganized general documentation and split into smaller chunks.
Change documentation syntax, delete legacy files
Change most remaining Doxygen tags

Moved some docs to README.md files as well.
Fix inline equations spanning two or more lines
Fix remaining @p tags
Remove @file tags
Fix plug-in docs, remove features plug-in
Implement JavaScript interface to main classes

Mostly reimplemented the functionality of the old script interface
with QJSEngine. Due to the lack of features in public Qt API, the
implementation is based on direct access to the underlying v8 API.

This is the first working version that makes it possible to create and
run PiiEngines in QML applications. Most of the features have not been
tested yet.

This commit also includes some refactoring to PiiResourceDatabase.
This was needed to make it possible to statically register
QMetaObjects to the resource database.
Refactor PiiSocket class hierarchy

QML cannot cope with non-QObject object hierarchies. Changed all socket
classes so that they derive directly from QObject. This is still a
completely broken commit that doesn't compile.
Fix build order issues with parallel make
Merge documentation, refactor socket classes

Merged remote-tracking branch documentation to qml-interface.

Finished refactoring socket classes. This commit compiles, but hasn't
been thoroughly tested yet.
Finish refactoring sockets for qml support

Fixed PiiPisoOperation's property protection levels at the same time.
Fix build order issues with make -j
Add PiiConfigurable support to PiiQObjectServer
Fix crash in deleting PiiOutputSocket
Fix most remaining bugs in qml support

The QML interface can now be seen as _mostly_ done. All C++ unit tests
pass. QML unit tests also pass except for reconfiguration, which needs
to be carefully designed. For now, property access from QML bypasses
the Into's protection mechanism and caching, which isn't acceptable in
the long run.
Implement image transfer from Into to QML

Added PiiQmlImageProvider and made some necessary changes to
supporting classes (such as PiiProbeInput and PiiQImage).
Merge branch 'qml-interface'

Branch 'documentation' was merged to 'qml-interface' first. Thus, this
commit merges both to 'master'.