Generalise filtering mechanism

Cleanup and simplify some other bits of the OXZ manager code
(next step: add some actual filters)
Offer to update OXZ list each time Oolite started and manager opened
Start of filter implementation

Also a better calculation of OXZ install status
Add keyword, author searches; adjust layouts
Add 'i' for information to OXZ manager

This allows additional paragraphs to be put into the description tag.
Before this is merged and people start being encouraged to use that for
extra readme information, the manifest API on the website needs to be
modified to only send the first paragraph of the description to Oolite/1.80
or earlier.
New 'x' option to automatically extract OXZs to OXPs
Adjust instructions
Add "installable" filter, make left/right arrow keys consistent
Add tag search and "last updated" search to filters
Merge pull request #111 from OoliteProject/oxz-manager-extras

Oxz manager extras