v0.4-xsymbol

This update to the original DirtyGutter plugin version 0.3b adds translation of Isabelle xsymbols upon reading a file when the buffer is using UTF-8-Unicode encoding, which stops spurious line changed indicators when, for example \<lambda> in the file clashes with λ in the buffer. It uses the xsymbol table exported from Isabelle 2023.

This fork/release is only of interest to Isabelle/JEdit users.