Change default toolbar actions
Based on the newly proposed update to the default Gwenview appearance (#5) this commit changes the actions that are placed on the toolbar by default. Because of further discussions in this MR, the toolbar contents were further refined.
The "go_previous" and "go_next" actions were added by me as a temporary workaround until the proposed floating buttons with the same purpose are implemented.
A class is added that is used to align toolbar contents with the sidebar (even if the sidebar is currently hidden).
Another toggle action was added that opens the "Operations" sidebar so we don't have to add all these important actions to the toolbar.
Hiding the menu bar by default is not part of this MR.