Use the current directory URI (set by the shell via OSC 7 escape
sequence) to determine the working directory for new tabs, instead of
reading /proc. This works inside tmux/screen/sudo, preserves symlink
components, and is consistent with other VTE-based terminal emulators.

The /proc-based detection is kept as a fallback for shells that don't
emit OSC 7 (e.g. vte.sh is not sourced). The initial working directory
is used as a final fallback, and the separate _with_fallback variant is
folded into the main function so all callers benefit from the complete
fallback chain.

Adapted from https://gitlab.gnome.org/GNOME/gnome-terminal/-/commit/cf3cad87

Fixes #195

The built-in color scheme dropdown always showed "Custom" when reopening
the profile editor. Two bugs were present:

- color comparison was called as pointer-to-pointer comparison, which
  always fails.

- gdk_rgba_equal does exact floating-point comparison, which is too
  strict for colors that have been round-tripped through GSettings
  string serialization. Using the fuzzy rgba_equal function fixes that.

Fixes #486