DatePicker: minimum and maximum dates

[rating89us]

Closes #2468

Main changes:

• "Date from" minimum date = Date of first transaction or date of Monero birth (2014-04-18) if wallet has 0 transactions
• "Date from" maximum date = "Date to" selected date
• "Date to" minimum date = "Date from" selected date
• "Date to" maximum date = Today
• Add "First transaction" and "Today" links on calendar bottom
• Mouse wheel considers minimum and maximum dates

[plowsof]

This is nice, the mouse wheel scroll respects the date limits imposed as promised. Today / First transaction dates work correctly.

The only weird thing i spotted (as shown in the 'Today' image) today is the 10th , but the highlighted day appears to be 11 (which is out of bounds , and not being drawn, this may be a timezone issue as its currently 2am on the 10th.. will check again tomorrow if its a bug or not)

Also, the orange text is going against the standard for clickable items, which are 'white' and change to orange when hovered over, i think we should keep with that. Or not .. would have to see it in white / bold to compare