add delete button to UI

[jabooth]

Would be nice to be able to delete all landmarks to start afresh
A delete icon in the UI would be useful for users who are less used to keyboard shortcuts.

[lirsacc]

Technically possible from v1.5.0 with the key combination a -> d or shift click on landmarks sidebar + d. (Select All, then Delete All)

Is that okay or did you have a one step thing in mind @jabooth ?

[jabooth]

no this is fine, although I do wonder if maybe we should add a delete button to the UI for discoverability? It's behavior would be the same as 'd', as in delete whatever is selected. Thoughts?

[lirsacc]

Could it be part of the group editing overlay we were talking about (see #97) ? Like a trash icon in a corner.