Use QPlainTextEdit for log window rather than QTextEdit

It is only used for plain text and is much more light weight that the
full text edit class.
Refs #7331
Refs #4858 Add spectrum range options to interface.
Refs #4858 Add validation to interface options.
Refs #4858 Hook options up to python code.
Refs #4858 Add sequential fit option.
Refs #4858 Hook sequential fit option up to python script.
Refs #4858 Don't plot graph with single data point.
Refs #4856 Fix "off by one" error breaking MSD system test
Refs #4856 Plot single fit directly in miniplot.
Refs #4856 Remove debugging statements.
Refs #4856 Remove fit plotting code.
Refs #4856 Update option to save when doing a single run.
Refs #4856 Add option to plot specific spectrum.
Refs #4856 Add correct x-units to plots.
Refs #4856 Correction to spectrum plotting logic.
Refs #4856 Swap single fit to use correct plot index.
Re #9030. Removed unused classes.
Merge remote-tracking branch 'origin/master' into feature/4858_msdfit_ui_upgrade
Refs #4856 Convert MSDFit to just use PlotPeakByLogValue.
Re #9030. Use covariance matrix to estimate errors of calc. values