Rework workaround to preserve dash visibility when switching workspace.

I was trying to raise the dash actor above the workspaceSwitcherPopup to
prevent it from stealing the focus and making a mess of the hover status. Now I
just ignore hover change when the worspaceSwitcherPopup exists.

The previous method seemed not to work always in recent gnome-shell versions.
Use gjs classes.

Just to update to the more recent syntax... no benefits I think.
Remove Bolt extension compatibility support code.

As the Bolt extensions is not going to be updated for new version of Gnome
Shell, I'm dropping the compaitibility code for it.
Intellihide code clean: simplify use of status variables.

Only use a _isEnable status to activate/deactivate intellihide callbacks.