[BUG] Partial charges input field cannot hold the entire JSON string

**Description of the error**
For trajectories with enough atoms, it will become impossible to set the charges in the GUI, as the JSON string becomes truncated.

Here I had 4000 atoms in the trajectory, but I could only set the charges for 2583 of them.

**Describe the expected result**
It should be possible to set the partial charges for any number of atoms.

**Describe the actual result**

The helper widget (above) shows all the charges to be set.
The actual input in the main widget ends suddenly:

**Suggested fix**
Temporarily, we could relax the limits on the maximum string length in the main widget's QLineEdit.

In the long run, it would also be good to switch to a more compact way of writing the charges.

**Additional details**
N/A

Start by locating the main widget's partial-charges QLineEdit and reproduce the truncation with a trajectory containing many atoms. Ensure the input can hold the complete JSON string shown by the helper widget, then verify that charges for all atoms can be set without truncation.

Written by the indexing model from the issue text.