TextBoxView uses DateTime.Now for time measurements

* .NET Core Version: All
* Windows version: 1903
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

**Problem description:**
` TextBoxView` uses `DateTime.Now` inside `FullMeasureTick` to measure a maximum time frame of 200 milliseconds for line measurements.
As `DateTime.Now` is "slow" and inaccurate a `Stopwatch` should be used and the time frame should be expressed as a `TimeSpan` instead of int or uint.
The same `TimeSpan` change should apply to the `_throttleBackgroundTimer`.