Toggle toolbar

• In normal mode — toggle header bar visibility.
• In fullscreen mode — toggle toolbar visibility.

On small screens maximizing available screen space is essential for enabling easy reading. This feature was already present in older versions (for normal mode), but was removed with UI rework in 3.8.

This patch series adds:

• menu item,
• right-click-menu item,
• keyboard shortcut (Shift+H) to toggle the toolbar,
• in fullscreen mode, auto hide toolbar when starting search or annotating.

Issue #336 (closed)

Issue #834

Bug #714996