API breaks during Tepl 6: Tepl 6 Reference Manual

## API breaks during Tepl 6

There are also ABI breaks not affecting the API, so just re-compiling the program is sufficient in that case.

Tepl 6.0 to Tepl 6.2:

- The `tepl_utils_override_font()` function has been renamed to tepl_utils_override_font_string().

Tepl 6.2 to Tepl 6.4:

- The `tepl_statusbar_*()` functions have been removed, you can use TeplLineColumnIndicator instead.

Tepl 6.4 to Tepl 6.6:

- Tepl now depends on libgedit-gtksourceview instead of GtkSourceView.

- The `TeplWidgetListItem` and `TeplWidgetListCategory` classes have been removed.

Tepl 6.6 to Tepl 6.8:

- The `TeplPanel` class has been renamed to TeplPanel1.

- The `TeplStack`, `TeplStackItem` and `TeplStackSwitcherMenu` classes have been removed. They are superseded by TeplPanel (and related classes and interfaces).

- The `TeplStatusbar` class has been removed, and is replaced by tepl_utils_setup_statusbar().