Refs #9041 Plot buttons reset appropriately, plus bugfixes

The plot buttons on a row will now reset to disabled when:
* The table is cleared
* A new table is loaded
* The table is reloaded
* A row is modified (this will only reset the modified row)
* In the rare case when the plot function fails to load cached data

There have also been a fair few bugfixes or improvements:
* File->Close Refl Gui now actually closes it, as until now it had loaded help.
* Clearing, reloading or opening a table will now do a modifed check and offer to save
* Clearing the table no longer sets the mdoified flag
* When processing all, only the lines with data in have their plot buttons enabled