Keep transform dropboxes up to date

The current transform dropboxes are confusing to me, because they often don't reflect the current transformation. For example after the following actions:

1. set "select axis" to signal
2. set "select xform" to log
3. set "select axis" to b-value

Now, the current setting shows "b-value" and "log", but the b-values are actually still linear (because we have not explicitly set a transform for it).

I propose the following updates:

1. Whenever a new axis is selected, the xform dropbox automatically updates to the transform currently selected for that axis (so in the example above it would switch to "none" after selecting the b-value".
2. Replace the "none" in "select xform" with "linear".
3. Remove the "none" option from the "select axis" box.

Proposals 2 and 3 are mainly, so that it is more obvious at a glance what these options are for. Currently, the dropboxes start out as "none" and "none", which gives no hint to the user of their use. With these changes they would start out as "signal" and "linear", which makes it much clearer what it is actually about.

I would also replace "xform" with "transform". It took me a little while to realise that that is what was meant with "xform".

Any thoughts @saad?