HTML improvements: HTTP cache, sender-dependent preferences

This merge request adds the following improvements to Balsa's HTML message implementation:

1. Activate the Webkit cache.
   This is in particular helpful if a message contains links to external material (typically images) which is downloaded only once. Unfortunately, Webkit does not offer methods for checking which items are already in the cache. Therefore, the question whether this material shall be downloaded is displayed even if everything is already in the cache. As the cache does not expire automatically, a “clear” button has been added to the Edit -> Preferences -> Settings… -> Message dialogue.
2. Implement sender-dependent HTML preferences (fix #13 (closed))
   A trivial SQLite database is added to remember for a given sender whether HTML should be preferred over plain text alternative parts, and whether external material should be loaded automatically. The options can be controlled using the context menu of HTML parts, and using the simple database browser reachable from the Edit -> Preferences -> Settings… -> Message dialogue. Note that SQlite ≥ 3.24 is required (available since e.g Debian Stretch backports, i.e. shouldn't be a blocker).

Note: Displaying HTML and in particular loading external material is always a risk at least for the user's privacy and should thus be activated for trusted senders exclusively. Remember that it is not possible to simply validate the sender address. The implementation just trusts the envelope, and cannot detect if it might be forged.

Signed-off-by: Albrecht Dreß albrecht.dress@arcor.de