app: improved algorithm for action search.

Now matches have various levels of reliability.
If the keyword is 2 characters, the match on first letters of first 2
words is most reliable; then a label prefix, then a substring anywhere
in the label; then a substring anywhere in the tooltip; and finally a
fuzzy match. This new algorithm would return the same list as before,
but in a much more significative order.
I also got rid of the " " = show all trick, replaced by a NULL keyword.
app: plug-in-recent-*/repeat/reshow filtered from action search.

plug-in-recent-* should not be historized nor shown, in particular since
they show the same label as the action they repeat and looks like
duplicated actions.
plug-in-repeat/reshow are not historized but are still shown in the list
because they have a different label and are still an interesting search.