Only save necessary info to folder preferences

[gave92]

Resolved / Related Issues
Items resolved / related issues by this PR.

• None, small optimization

Currently we are serializing unnecessary/calculated fields of ColumnsViewModel to the preference DB. This PR adds [BsonIgnore] to those.

Before (each column per each folder):

"NameColumn": 
{
  "LengthIncludingGridSplitter": 
  {
    "Value": 248.0,
    "GridUnitType": "Pixel",
    "IsAbsolute": true,
    "IsAuto": false,
    "IsStar": false
  },
  "Length": 
  {
    "Value": 240.0,
    "GridUnitType": "Pixel",
    "IsAbsolute": true,
    "IsAuto": false,
    "IsStar": false
  },
  "UserCollapsed": false,
  "UserLength": 
  {
    "Value": 240.0,
    "GridUnitType": "Pixel",
    "IsAbsolute": true,
    "IsAuto": false,
    "IsStar": false
  },
  "UserLengthPixels": 240.0
},

After:

"NameColumn": 
{
  "UserCollapsed": false,
  "UserLengthPixels": 240.0
}

Validation
How did you test these changes?

• Built and ran the app

[yaira2]

LGTM