Synth-mapping-helper

When uploading .synth files directly output by SMH, they would be refused with an internal server error.

Now they work, which required explicitly encoding one part in a very specific way (which is notably different from another part of the same type).

Fix typo preventing filters from working and an issue interpolating extremely short rails.

* Filter can now by quickly disabled completely, or disable type and rail filters separately.
* Single notes are no longer 0-length rails. but a separate filter option. This avoids the intuitive interaction with splitting rails (or smoothing) as mentioned in the last release.
* You can filter by node-count now too
Note that in this screenshot, the filter matches all rails that have at least 3 nodes (including rail head) **AND** node spacing between 1/16 and 1/4. Rails must match **ALL** configured filters (not just one).

Dashboard Filters
You can now filter by:
* Type. You can also quickly invert the type selection using the toggle above. I hope the icons are intuitive.
* Rail length. Note that **single notes count as 0-length rail**, so setting the "Min" to above 0 will also ignore all single notes, which makes some functionality not work, like splitting rails
* Rail spacing. This is fairly useful when you want to ignore buzzrails or long straight rails for smoothing.

Let me know if the filter doesn't work as expected for any operation (I tested many combinations, but obviously not everything).

I also updated the rest of the top bar to be more icons+tooltips and less visible text, and there is now a "pick pivot" button.

Snap-Smoothing
There are now three modes for smoothing:
* "Snap" (Arrow symbol) first smoothes the rail and then snaps single notes to the smoothed rail
* "Anchor" ensures the smoothed rail passes through all single notes
* "Tempoary Anchors" as above, but the single notes are auto-removed (this assumes you placed temporary anchor notes you want removed)

When saving .synth file (e.g. via File-Utils), SMH always and automatically adds a hash to the audio filename, to force the editor to re-generate the audio cache ("fixing" the "replacing the song file doesn't work" bug).
However with the hash added, the filename might be to long for the way the game handles unpacking, so this hash is now much shorter.

Also fix hand-curves crashing when there was no velocity/acceleration data, e.g. if there was just a single note of a type.

There is now a "smoothing" section in the dashboards, and some other sections were reordered and some tooltips expanded to make some more obsure functionailty more intuitive.

Below is a screenshot of smoothing: in red, a rail quickly placed with grid snap enabled, and in green the output after smoothing:


Smoothing works by "deleting" every second node, interpolating them based on the remaining nodes and averaging old and new position according to the "resistance". This process is then repeated in the second iteration with the other set of nodes, and then it starts over.
Higher resistance makes less pronounced changes and keeps more of the original shape, while more iterations will lead to a smoother result, which may just be a straight line with enough iterations.
Rail start, end and any single notes on the rails will be treated as anchors, to avoid it straightening too much or enforce certain shapes.