Placeables: Use Translate Toolkit built from repo instead of package

This is necessary since the latest Translate Toolkit package doesn't include
some changes done to placeables code that might be necessary to use.
Placeables: Highlight and copy with mouse click in editor

Fixes bug 3146
Placeables: Add tooltip indicating type of placeable
Placeables: Replace fancy_highlight with hightlight_placeables templatetag
Add a dialog with the keyboard shortcuts list in the editor page

Fixes bug 2589
UNDO 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.
UNDO 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.
UNDO 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.
UNDO