GUI 1d data loader: Support multiple formats; store data, not loaders; make configurable-loader dialog modal

Currently, the 1D loader GUI is all about loading tabular 1D data with arbitrary, unknown layout, but makes little sense and is not easily adapted when there are also established data formats like MFT (#591 (closed)) to be supported. In perspective, users should migrate to an established data format. Therefore, we should make it as easy as possible to load data in established formats, and we may well make it a bit less convenient to load undocumented tables.

• rm "user defined" loader (this is incomplete, unused code) - done in !1646 (merged)
• replace QREDataLoaderProperties.ui by cpp code (contributes to #513 (closed)) - done in !1647 (merged)
• split the loader properties widget from the loaded data view, and make it modal - done in !1649 (merged)
• store loaded data, not loaders - done in !1648 (merged)
• make MFT loader accessible from GUI - done in !1648 (merged)