Snapshots: update view snapshot system after MDI is removed

[jonjenssen]

When using the dock system without MDI, getting snapshots the old way with a given window size no longer works.

• Support setting image size from command line
• Support snapshots from ResInsight user interface
• Support snapshots for regressions tests
• Update Python interface to allow user to specify image size

[jonjenssen]

Methods to look at:

RiaRegressionTestRunner::setDefaultSnapshotSizeFor3dViews()
RiaRegressionTestRunner::setDefaultSnapshotSizeForPlotWindows()

RicfExportSnapshots::execute()

RicSnapshot*

RiaGuiApplication::handleArguments(...)