Sketcher/Gui: Extend toolbar framework and fix toolbar visibility not preserved

=================================================================================

fixes #9208

Essentially:
- The regular mechanism to save toolbar state when changing from one WB to another
is not designed to support changes within one WB (e.g. from edit mode and back).
- At creation time, toolbars can be initialised with default visible or default hidden
state. Additionally, there is third configuration "Unavailable", which refers to a
toolbar that is hidden, and the control to enable it is also hidden by default.
- The ToolBarManager is extended to enable to set the State of one or more toolbars.
- The State refers to changes to be effected on one or more toolbars by client code:
* ForceHidden allows to hide a toolbar and also hide its control (a toolbar not available
in a mode).
* ForceAvailable allows to make a toolbar available by making its control visible, the
toolbar itself is visible or not depending on user settings.
* RestoreDefault allows to bring the control visibility to the default of the toolbar, the
toolbar itself is visible or not depending on user settings.
* SaveState allows to store the current visibility state of a toolbar. It enables client
code to save the state when appropriate. It provides the only option for default "Unavailable"
toolbars, which are fully managed by client code. It provides additional flexibility to save
other toolbar visibility on request.

For the Sketcher this means:
- That edit mode toolbars are not shown outside edit mode.
- That edit mode toolbars and non-edit mode toolbars can be configured independently.
- that edit mode toolbars' state is saved when leaving edit mode if and only if, the
workbench that is selected when leaving edit mode is the Sketcher WB.
- it won't save the state if the user manually selected another WB and then left edit
mode (why? see limitation above).

Limitation:
- When switching to another WB while in edit mode, the other WB is activated before the
current WB (sketcher WB) is deactivated. This means that at sketcher level, the sketcher
has no chance to save states or do other tidy up actions before the tools of the other WB
are activated.
- This, however, is understood as not relevant enough as to warrant changing the mechanisms
in place.

src/Gui/Workbench.cpp

@@ -774,7 +774,7 @@ ToolBarItem* StdWorkbench::setupToolBars() const
           << "Separator" << "Std_Refresh";
     
     // Clipboard
-    auto clipboard = new ToolBarItem( root , ToolBarItem::HideStyle::HIDDEN );
+    auto clipboard = new ToolBarItem( root , ToolBarItem::DefaultVisibility::Hidden );
     clipboard->setCommand("Clipboard");
     *clipboard << "Std_Cut" << "Std_Copy" << "Std_Paste";