Gdbgui

**Breaking Changes**

- Removed support for Windows
- Replaced `--gdb` flag with `--gdb-cmd`. The `--gdb-cmd` argument specifies the gdb executable as well as all arguments you wish to pass to gdb at startup, for example `--gdb-cmd "gdb -nx"`. The existing `-g` argument is an alias for `--gdb-cmd`.
- Removed `--rr` flag. Use `--gdb-cmd "rr replay --"` instead.
- Removed deprecated and hidden `--hide-gdbgui-upgrades` argument. It will now raise an error.

**Additional Changes**

- Replaced single terminal on frontend with three terminals: an interactive xterm terminal running gdb, a gdbgui console for diagnostic messages, and a terminal connected to the inferior application being debugged.
- Updates to the dashboard
- Add ability to specify gdb command from the browser. This can now be accomplished from the dashboard.
- Removed gdbgui binaries from source control. They can now be downloaded as artifacts of [releases](https://github.com/cs01/gdbgui/releases).
- [documentation] Fix bug when generating md5 checksum for binary releases
- Remove "shutdown" button in UI

- No end user changes. This release builds the gdbgui executables with GitHub actions.

- Print number of times a breakpoint was hit (MatthiasKreileder).
- Publish sdist to PyPI (this was overlooked in previous release).
- Do not notify users of gdbgui upgrades (deprecate `--hide-gdbgui-upgrades` flag)
- Drop support for Python 3.4
- [dev] Some infrastructure changes to gdbgui. End users should not be affected.
- [dev] Fix build error due to webpack bug (https://github.com/webpack/webpack/issues/8082).

- Exclude "tests" directory from Python package
- Remove analytics from documentation

- Add `__main__` entrypoint

- Remove automatic flushing of stdout and require newer version of pygdbmi
- Add flake8 tests to CI build