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
UNDO Add in editor a dialog with keyboard shortcuts list

Fixes bug 2589
DON'T MERGE: Use TTK from repo to test placeables
Cleanup: Move form and formset from pootle_project views to forms.py

This helps to keep view code simpler.