Marble/Marble 2.0

Refactoring goals towards a Marble 2.0 version.

Marble Library Split

Split libmarblewidget into several libraries:

libMarbleCore (or libMarbleMap?)
- Non-interactive classes, e.g. geodata, file parsing
- Interactive, UI toolkit independent classes, e.g. layers, graphicsview
- No dependency to QWidgets or Qt Quick
libMarbleWidgets
- QWidget related interactive classes
- Dependency to QWidget, libMarbleCore
libMarbleQuick
- Qt Quick 2.0 related interactive classes
- Dependency to QtQuick, libMarbleCore
Further possible ones (see also plugins below): MarbleRouting, MarbleTours, MarbleMapEditor

During compilation, the subdirectories of marble/ are not added to the compiler search path anymore, so #include statements need to use the full path marble/$module/$class.h for files outside their directory.
Headers follow the directory structure marble/$module/$path, where $path is either a filename or a directory + filename, but not deeper nested

The header structure in src/lib/ should match the exported headers in include/. No "flattening" as it happens currently where e.g. marble/geodata/* is copied to /marble/.
KDE as a dependency is only used in src/apps/marble-kde and KDE related things like plasmoids. No KDE usage in the library. As a side effect, QTONLY will go away (replaced by a less restrictive WITH_KDE) Implemented since Applications/15.08
Provide a sane FindMarble.cmake, possibly using COMPONENTS similar to Qt, Boost (each library would be a component)

Require at least Qt 5.0 to avoid a maintenance nightmare
~~Require KDE Frameworks 5 (optional, but kdelibs 4.x not supported)~~ Implemented since Applications/15.08
Require Qt Quick 2.0 (optional, but Qt Quick 1.1 not supported)
~~Provide a long term support stable branch with Qt 4.x / KDE 4.x / Qt Quick 1.1 support~~ Please use Applications/15.04 for that

Clearly split between the loading mechanism and the interface (API) such that extensions can be realized both inside a library and via plugins without hassles (emphasis on without hassles)
Deleting plugins should be possible and remove the feature provided by it, but it should not be advertized as a feature
No mandatory plugins (e.g. KML parsing). Mandatory "plugins" must be part of a library
Decide on criteria what to realize as plugins and what as modules (libraries). E.g. only do plugins where a common API is easily identified and at least three plugins of that type are provided by Marble itself