allow to store project settings

[janzandr]

Discussed with the team and requested by multiple users. Is kind of a hot topic. Several users didn't want to use the EnMAP-Box GUI, because complex GUI setups get lost after every crash or after closing the EnMAP-Box.

Replaces issue on bitbucket: https://bitbucket.org/hu-geomatics/enmap-box/issues/36/project-settings

• [x] save EnMAP-Box settings as part of QGIS Project (*.qgs file):
• [x] implement an EnMAPBoxProjectSettings class managing all settings
• [x] implement a mechanism that allows Core/User Apps to get/set individual project settings
• [x] implement a mechanism that allows QgsDockWidgets to get/set individual project settings
• [x] project settings are stored and restored using the QGIS menu buttons; replicate those buttons inside the EnMAP-Box project menu
• [x] provide two interfaces for getting/setting project settings: 1) via dictionary, 2) via QDomDocument

[ernstste]

Ideally, this would include all layer styles, plot options, etc..., that make up complex setups (e.g., for reference data generation) to keep the time required for setting up the workspace to a minimum after closing the project or a crash.

[janzandr]

@jakimowb, an EnMAPBoxApplication has now access to the QGIS DOM Document and the EnMAP-Box DOM Element:

@jakimowb, for now, I wouldn't introduce more structure into the XML. You are probably the only one using the direct XML read/write anyways. Maybe at a later stage, if we really need it.