This is necessary in order to be able to correctly translate between absolute and relative paths
deeper in the code - e.g. paths to SVG files used in marker or fill symbols.
Until now, relative paths were translated to absolute paths on the fly.
This is now changed - paths to files should be always absolute within QGIS objects - and paths
only get turned into relative when saving projects. When loading a project, relative paths
are translated to absolute paths immediately.
This should lower the overall confusion about relative/absolute paths within QGIS, and also
allow having different base directories for relative paths (e.g. QML or QPT files may use relative paths
to their directory - rather than to the project directory)
This commit implements a new QgsAnnotationManager class, which
handles storage, writing and retrieval of annotations.
QgsProject has an annotationManager() attached to it. Map canvases
sync their visible QgsMapCanvasAnnotationItems to the annotations
contained within the project's annotation manager.
This moves all management, storage and retrieval of annotations
up to core and out of app/canvas.
