6.0 will be in the field for a long time, and the better the debugging
tools we have available the less costly it will be to maintain.
Fixes https://gitlab.com/kicad/code/kicad/issues/8961
CHANGED: Highlight net tool changed to highlight display toggle; moved to left toolbar
CHANGED: Multi-net highlight is now accessible when using the highlight net action
ADDED: Highlight Net and Clear Highlight actions to Net Tools context menu
ADDED: Default hotkey (Ctrl+`) for toggling net highlighting
The current and previous net highlight state are now stored and can be toggled
on or off with an action (no hotkey by default).
Highlight is now treated as a persistent state rather than a tool that is entered,
meaning there is no longer a special tool to enter in order to highlight nets.
Highlighting can be controlled while any other tool is active using the hotkeys
or context menu on a net object.
Tweak the icons for highlight net and layer display mode to better disambiguate them.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/8250
Jeff Young
Marek Roszko
Jon Evans
Wayne Stambaugh