Make sure modifier keys work

Feature: https://source.puri.sm/Librem5/Apps_Issues/issues/65

Using modifier keys results in nothing the keys changing labels. May not be so important if #7 (closed) goes in first.

Under the new design, modifiers are reduced to mean only keys that don't change the emitted text, i.e.: not shift, not caps. They can deped on what the layout creator wants, but in general it's those:

• Ctrl
• Alt (not Gr)
• Command

If any of these are enabled, text input should be disabled and raw keys should be emitted.