Refactoring of mail header view.

Refactoring of mail header view.
* Refactored the mail header view. Now we have the interface
  ModestMailHeaderView providing support for alternate implementations.
* Now old ModestMailHeaderView is ModestCompactMailHeaderView
* Added new ModestCompactMailHeaderView, with a style made by labels.
* Added signal show details to mail header view hierarchy
  (implementation for signal emition only in compact view).
* Some code cleanups and style enhancements.

Loading message after showing message view:
* Added methods for loading the messages in message view _after_ view
  is shown. We use the code in message_reader already available.

Other changes:
* Proper bold/greying of elements in folder view.

pmo-drop-split-view-r6298