Allowing range entry of entry memory

[riesentoaster]

• Closes #4
• When the end address is smaller then the start address an error is shown
• To add just one entry: Make end address equal to start address

[riesentoaster]

I added error messages if

• The start address is empty
• The value is empty
• The register name is empty
• An error occurred while parsing the hex values (should never happen in theory because the HexDocument only allows valid hex chars, but y'know…)

If the end address is empty, it is set to the same value as the start address (good idea!).