Add shortcuts informations in the settings view

[fraxken]

I think we should add some text in the settings views to explain all the available shortcuts (as we probably plan to add more of them in short term).

Current shortcuts are:

• H (home view)
• N (network view)
• S (settings view)

We can add a section "shortcuts" (below the save button of General section). Raw text with a list is enough for a V1 (but later we can imagine that we will be able to customize them?).