Add a dialog with the keyboard shortcuts list in the editor page

Fixes bug 2589
Placeables: Cycle forward through existing placeables using keyboard shortcut

Mimics Virtaal behavior:

* If the last placeable is selected then all placeables are unselected,
* If no placeable is selected then selects the first one,
* If any other placeable is selected then selects the next one.
Placeables: Cycle back through existing placeables using keyboard shortcut

Mimics Virtaal behavior:

* If the first placeable is selected then all placeables are unselected,
* If no placeable is selected then selects the last one,
* If any other placeable is selected then selects the previous one.
Placeables: Copy placeable to translation field using keyboard shortcut

Mimics Virtaal behavior:

* If there is a selected placeable then copy it to the translation field, in
  the position where the cursor is, or at the beginning of the line if there
  is no cursor.