1.7 Display Unit changes

A number of related changes to the units used in the GUI. These units are used for display, for sending coins and for the send confirmation dialog, which shows the amount in all avalable units.

This removes milliDoge and microDoge as options since I see little use for them. For use cases like finance software that can't cope with more than two digits after the decimal seperator, Koinu is instead introduced. 1 Koinu equals 1/100th microDoge, making it a Satoshi equivalent.

Users using MilliDoge or MicroDoge as the current setting will fall back to Doge with this change.

MegaDoge and KiloDoge are introduced as new units. They are are especally useful for sending large amounts, as discussed in #365. Now you can just send 2 MDoge instead of typing an amount of 2000000 Doge and counting zeros.