Add QLineEdit support to UISliderWidget

[jcornall]

Closes #125 #60

The current UISliderWidget is limited due to the following factors:

• Instantiating a UISliderWidget requires a QLabel, which is updated to show the value of the slider
• The QLabel cannot be updated manually
• The slider does not support float/double values

This pull request removes the QLabel parameter in favour of a QDoubleSpinBox parameter, which displays the current value of the slider, can be manually edited (supports float/double values), and updates when the slider value is changed.

An example has been included in the /examples/ folder to demonstrate the functionality.

PLEASE NOTE: By default the QSlider.value() only supports integer values - even if the value of the QDoubleSpinBox is a float/double, this value is automatically converted to an integer when the slider value is updated. I am investigating solutions to this problem so that the slider value accurately reflects that of the QDoubleSpinBox.

Current Tasks:

• Implement QLineEdit
• Remove hardcoded values
• Make sure spinbox is linked to slider
• Overload the init, one with the DSpinBox and one without
• Add min and max QLabels
• Consider whether to split the class into two separate classes
• Add tests for the widget to eqt

[jcornall]

Updated PR title to reflect changes to the design of the UISliderWidget class.

Following discussions with @paskino and @DanicaSTFC, it was agreed that instead of a QDoubleSpinBox the UISliderWidget class would make better use of a QLineEdit, removing restrictions on the decimal values users can input.

[jcornall]

QSlider 1: Modified UISliderWidget, individual widgets passed as arguments to connect them to the QSlider, then added to the FormDialog afterwards.
QSlider 2: New class UISliderLEditWidget, which creates its own widgets and adds them to a layout for display.