Fetch features in background threads

[uclaros]

This PR fixes #2211
There were two reasons for the slowness:

1. Searching was done on the main thread, and the search time depends on totalFields x totalFeatures
2. Numeric fields are checked using the very slow ~ regex operator, which makes 1. even worse

To mitigate this:

• layer.getFeatures() is ran on a background thread using QtConcurent::run()
• searching against numeric fields is changed from using the regex operator ~ which is super slow to the ilike operator.

Drawbacks:
#2045 may be easier to hit, as with adequate number of fields + fields names length sqlite hits a parser stack overflow and we get no results at all.
We can address this on a separate PR by requesting all features from the layer and do the matching ourselves.
In that case, the request will only include the relation related expression and not the search filter expression.

[tomasMizera]

Hi @uclaros, can you please rebase this PR? You will get a new certificate for iOS and its build should work fine then