Reference signal is not kept when disabling and re-enabling "Mark diffs in protocol"

[Nesos-ita]

Expected Behavior

The selected reference signal is kept, for example if i have 7 rows and i click the 3° as reference, when disabling and re enabling mark diff it should be still the 3°

Actual Behavior

The reference signal is always the first row when i enable the checkbox.
If i hide some rows and keep only the 3° and the 5° rows the reference signal is yet the 1° row, even if it's hidden, this should be fixed too as noone probably want to compare two rows against one that has been intentionally hidden. (initially i thought about a bug in the diff itself, and only after i found out that it was the wrong reference signal)

Steps to Reproduce the Problem

0. [optional] hide some rows, and the first row
1. Enable mark diff
2. Select a row as reference (right click->mark as reference) (not the 1° row)
3. Disable mark diff
4. Enable mark diff, the reference is 1° row (absolute number, not the first visible)

Platform Specifications

• Python Version: installed 3.6 but it uses 3.5 included with the msi
• Operating System: win8.1 x64
• Version of URH: 2.0.4 MSI installer x64
• URH was installed: via msi