Minor design tweaks to the main menu

• Apply header capitalization to tooltips
• Adds tooltip for the zoom in/out buttons
• Adds ellipsis to the "Print" menu entry
• Changes the a11y label of the "Main Menu" button to "Main Menu" (previously it was just "Menu")

See details:

• https://developer.gnome.org/hig/patterns/feedback/tooltips.html?highlight=tooltips
• https://developer.gnome.org/hig/guidelines/writing-style.html?highlight=ellipsis