Add 'Ctrl-5' keyboard shortcut for 'strikethrough' #1987

A quick search revealed that 'Ctrl-5' is a popular shortcut
for strikethrough. Implemented in 'ace2_inner' and updated
the English l10n file for the tooltip.
Merge pull request #2265 from prtksxna/keyboard-shortcut

Add 'Ctrl-5' keyboard shortcut for 'strikethrough' #1987
Add keyboard shortcuts for ordered and unordered lists and update tooltips #1987

Ordered List: Ctrl-Shift-N
Unordered List: Ctrl-Shift-L