This includes the following change:

- Moving all icons to `Markups/Resources/Icons`
- Renaming `MarkupsMouse...` by `MarkupsFiducialMouse...` to keep naming consistency
- Adding a new `MarkupsGeneric...` set of icons

This allows the creation and registration of markups in modules other
than the Markups module

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Co-authored-by: Andras Lasso <lasso@queensu.ca>
Co-authored-by: Kyle Sunderland <sunderlandkyl@gmail.com>
Co-authored-by: Steve Pieper <pieper@isomics.com>
Co-authored-by: Csaba Pinter <pinter.csaba@gmail.com>

This extends the markups labels naming by adding a short-name version of
the labels. This short-name version of the labeling is established as
default. Disambiguation of open and close curves is achieved by setting
the label to 'OC' and 'CC' respectively.

The reason for this change is to bring consistency in the naming of
markups. Before this commit, placing markups from the selection node
toolbar will use short naming, while placing markups from the Markups
module GUI would use long names.

This closes Slicer#5196.

This renames qMRMLMarkupsROIWidget to qSlicerMarkupsROIWidget in order
to keep naming consistency across markups widgets.

This adds a new test file to validate the functionality of pluggable
makups on `vtkMRMLMarkupsLogic`

This adds unit testing for `qSlicerMarkupsAdditionalOptionsWidgetsFactory`