Implemented register view

Import project file is broken (texts and registers);
Still need to add data checks at start;
Fixed project file import

Also added "active" attribute to var node;
Add checks for duplicate registers

Check is done when starting scope and when importing project file;
New line is set to next free address;
Allow all modbus slave id's

Normal slave id's range from 0 to 247. But slave id's 248 to 255 are
sometimes used to access administration/testing registers. So this
should also be allowed.
Remove current value from graph name when exporting to file
Fixed data export (very large times)

Scientific notation is not used anymore on very large numbers;
Data export now clears file before writing new data;
Improved poll interval scheduling

Make sure the poll interval is the time between the start of 2 polls and
not the time between end of poll and start of next poll;