general: Tweak tooltips and menu items to follow HIG

This MR applies:

• Header capitalization to tooltips and menus
• Makes tooltips shorter
• Adds a tooltip to the "Main Menu" button

See also:

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