Custom render profiles makes all optional settings mandatory & unable to edit the text string

This bug is specifically related to the custom profiles that one is able to create in Kdenlive now. It was hinted at in #1439, and other bug reports, but here it is spelled out without any other issues to confuse things.

1. It is impossible to edit the text part of a custom profile from the GUI.
If I was able to edit the text of the filter, I could fix every single issue I have with creating custom filters

2. Every single option that should be optional is being forcefully included. 
2.1 Output resolution needs to be optional. Sometimes I work in HD, sometimes I work in 4K, why do I need to make two almost identical profiles?
2.2 Pixel aspect ratio is only necessary when it is not 1:1
2.3 Display aspect ration is only necessary when it is not 1:1
2.4 Frame rate should come from the project, unless one wants to change it.
2.5 Scanning should come from the project, unless one wants to change it.
2.6 Field order is only valid for interlaced video, also comes from project.
2.7 Bitrate needs to be adjustable just like in the profiles already included in kdenlive
2.8 Buffer size? What maniac sets this different from the input video?
2.9 Quality and Bitrate can't be specified together
2.10 GOP - Really? This should come from the project.
2.11 B-Frames - Same as 2.10.

Am I misunderstanding the purpose of the custom render profile? In the past I was testing hardware accels, CUDA and all sorts of funky stuff, and I feel that the current implementation of custom render profiles is not only flawed, but severely limiting.

A quick fix for the custom profiles would be to allow one to edit the text field of the filter, and to make every single option on all the tab be possible to be toggled on or off.

As an example use case: Let's say I WANT to make a render profile that specifies the output resolution, I could click on the toggle on a button next to resolution, and then the bit of text that is needed in the profile is added to the text string. The button should stay lit, unless It is unticked, and then the text should be removed from the render string. Also, if I manually remove that text from the render string, the toggle should also be un-ticked.

I should also be able to completely re-write the render string, so that I can test new and unusual render profiles.