PeakCombo now only shows collections associated with current DataSet

The goal of this MR is to iron out all of the bugs associated with processing and merging multiple runs in a single experiment. The main change is to associate PeakCollection, UnitCell and ShapeModel with a DataSet, and ensure that such objects can only be selected via the combo boxes if they are associated with the current data set (which is a property of the GUI element Project). Note that since UnitCell has a pointer to a data set as a member, this must be saved in the ohkl file, and existing ohkl files will no longer be compatible.

Renamed some GUI elements: SubframeMergedPeaks -> SubframeMerge, SubframePredictPeaks -> SubframePredict, SubframeFilterPeaks -> SubframeFilter for conciseness.

Updated several refresh mechanisms in the GUI, mainly in SideBar and the subframes, to avoid any potential unwanted recursive calls.

Change the peak exporting mechanism in SubframeMerge to use PeakExportDialog instead of its own method.

Resolves #646 (closed), #645 (closed), #527 (closed), #530 (closed), #647 (closed)