shell: Make menu entries GNOME HIG compatible
GNOME HIG suggests using Header Capitalization for menu entries.
Header Capitalization
Header capitalization should be used for any headings, including headings in header bars, tab titles, and view titles. It should also be used for short control labels that do not normally form proper sentences, such as button labels, switch labels and menu items.
https://developer.gnome.org/hig/guidelines/writing-style.html#capitalization
Edited by Sabri Ünal