Review fallback location for temporary files

[AlisterH]

Undo files and the clipboard data are now saved in the current working directory if $XDG_CACHE_HOME is not available.
If CWD is read-only this does not work, so it would probably make more sense to fallback to the directory that contains the file we are working on, as we can't work anyway if that is read-only.

[AlisterH]

Also note that "Copy" fails silently if CWD is read-only - if it runs out of space or something would the failure also be silent?
EDIT - I've added an error message so it isn't silent

[AlisterH]

If CWD is read-only this does not work

This is the case on OSX when running from a .dmg
But we can avoid a problem there by simply setting the environment variable in the startup script.

[AlisterH]

If CWD is read-only this does not work

This is the case on OSX when running from a .dmg
But we can avoid a problem there by simply setting the environment variable in the startup script.

Done in git.

[AlisterH]

Changes committed to git show dialogs with the warning messages, so at least people shouldn't get into trouble with this now.

[AlisterH]

Should probably stop saving noise.dat in CWD, too.

[AlisterH]

Should probably stop saving noise.dat in CWD, too.

Should actually display a graph or something instead: #23 done