Optimize side-panel-handler.

Currently, side-panel-handler makes lot of needless calls to empress's resetTree/coloring functions. For example, every time the user changes line width, side-panel-handler will reset the tree, then recolor it (and potentially re-collapse), and then finally adjust the line thickness. However, adjusting line thickness can be done without resetting the tree. I don't think this is a huge issue right now. However, this may slow things down with larger trees.

It would also be nice to remove the update button.

Start with side-panel-handler and trace its calls to empress's resetTree and coloring functions, especially when line width changes. Check how the update button is wired and identify whether both optimization and button removal belong in this issue. Done means line-width changes avoid unnecessary tree resets while preserving behavior, and the update button is removed if its behavior is in scope.

Written by the indexing model from the issue text.