Add support for indented lines to TSVSerialiser.

The <spectrogram> section indents its subelements. TSVSerialiser doesn't
parse that correctly. This commit adds support to parse indented sections
correctly.

Refs #9970
Remove orphaned method Graph::restoreSpectrogram.

Refs #9970
Stub IProjectSerialisable interface in Spectrogram.

Refs #9970
Improve multilayer scale parsing.

Axis scales were not being loaded correctly. This has *mostly* fixed it.
When loading spectrograms (and probably other plots) the right Y axis
scale is not being set correctly, and nor are the major tick values on
any of the axes.

Refs #9970
Re-enable restoring geometry of most windows on project load.

Refs #9970