Streamline our custom Label/LineEdit system

[matty0ung]

Broadly speaking, the idea was to create two new classes, SmartLabel and SmartLineEdit replace the inheritance hierarchies of BtLabel and BtLineEdit. Having to create a new (albeit trivial) subclass of each of these for each new type of thing we want to measure was becoming unwieldy. Also, as @mikfire has previously noted, it's a pain to have to remember which version of BtLineEdit etc to use where in the .ui file.

There is slightly more to it, specifically:

• BtLabel and all its subclasses (BtColorLabel, BtDensityLabel, etc) -> widgets/SmartLabel
• BtLineEdit and all its subclasses (BtGenericEdit, BtMassEdit, etc) -> widgets/SmartLineEdit
• BtDigitWidget and its subclasses (BtMassDigit, BtGenericDigit) -> widgets/SmartDigitWidget
• UiAmountWithUnits -> SmartField (base class for SmartLineEdit and SmartDigitWidget)
• New helper functions in SmartAmounts namespace, plus macros to call them

Various bits of .ui files are simplified as, eg, signal connections are done in code now.

This should fix #733

Also includes fixes for #741 and #743