dev: Correct some minor inconsistencies in the UI

[t52ta6ek]

Control panel width increased a few pixels to allow Markers -> "reference" checkbox to be capitalized like the other checkboxes and radio buttons. Now able to be displayed as "Reference" without cutting off the text of the "Enable Delta Marker" checkbox.

Added icon_48x48.ico so that pyinstaller/auto-py-to-exe can create a Windows executable with the actual NanoVNA-Saver project icon instead of a generic Python icon. The .ico was created by converting the icon_48x48.png image.

"Files" button in control panel changed to "Files ..." to be consistent with other "..." actions where a user will take additional action after pressing the button.

Minor corrections in the Calibration page "Are you sure?" and "Calibration assistant" pop-ups where text was being run together. e.g. "doso" instead of "do so" etc.

Fix issue: #663