Keymap mod keys: Move ConflictView to top.

Also, move includes from .h to .cpp file.

Make _FillSavedIcon() private, make Draw() public.
Keymap mod keys: style fixes
Keymap mod keys: Use menu field label and refactor

Use CreateLabelLayoutItem() instead of faking the label with a BStringView.
This is important because it means the label is part of the menu field control
rather than being it's own separate view.

Also convert from using deprecated BGroupLayoutBuilder and
BGridLayoutBuilder classes and use the BLayoutBuilder template instead.

Use font-relative spacing values instead of constants.

Make key label take up 2 columns

Make protected methods private, don't want to subclass
ModifierKeysWindow
Keymap mod keys: Conflict if either left or right

... key doesn't match rather than requiring both left and right keys
to not match for a conflict.

Also update copyright year