Allow to merge simple filters

[spotlesscoder]

Prerequisites

• Tried the most recent nightly build
• Checked if your issue is already reported.

Description

Assume we have two filters:

• When message subject contains "hello" or "world"", delete message
• When message subject contains "hello" or "github", delete message

I want to merge those two to a single filter:

• When message subject contains "hello" or "world" or "github", delete message

So that means

• if filters contain only "or" statements on the same field and do the same action on a match, a new filter with the exact same action and distinct keywords should be created and the old filters should be removed. That should help reducing the amount of filters