Add reload functionality
Turn (edb-cur-entry) into a string.
Rewrite filter functions.
Use edb-keys-list rather than edb-cur-entry to check if there are entries.
Use completion when filtering on keywords.
Add more filtering functionality.

- ebib-filter-reapply
- write filtered entries to new db without renaming original db
Pretty-print filter.
Do not fill index buffer if no entry matches the filter.
Move ebib-run-filter to other filter functions.
Do not fill entry buffer if no entry matches the filter.
Move ebib-redisplay and ebib-fill-index-buffer.
Fix manual.

- replace all references to "virtual database" with references to
  filters.
- correct list of key bindings for the entry buffer.
Set ebib-cur-keys-list to nil when last db is closed.
Allow editing filtered entries from the menu.
Use ebib-cur-keys-list in ebib-execute-helper.

The `entries' option of ebib-execute-helper now checks
ebib-cur-keys-list.
Remove `and' in entries clause in ebib-execute-helper
Store and save filters.
Fix docstrings.
Set default filter file to ~/.emacs.d/ebib-filters
Honour ebib-filters-ignore-case in ebib-filters-select-filter.