Add combobox to Windows

[Phillips126]

Here is an implementation of a combobox feature for NiGui (Windows only).

Comboboxes can have several styles: CBS_SIMPLE, CBS_DROPDOWN, or CBS_DROPDOWNLIST so I included a way to pass the desired type when creating the combobox. Possible values are: simple (default), dropdown, dropdownlist. Leaving this unspecified will default to CBS_SIMPLE

Creating a combobox:
var combobox = newComboBox(@["Choice 1", "Choice 2"], "dropdown")
var combobox2 = newComboBox() (will create empty combobox)

I've created a few methods as well for combobox interactions:

• getOptionCount(): returns number of options currently available in dropdown
• addOption(<string>): adds a new option to bottom of dropdown
• deleteOption(<int>): deletes option from list at index specified
• selectOption(<int>): selects option at index
• findOptionIndex(<string>): searches list of options, returns index or -1 (not found)
• getSelectedIndex(): returns the index of the selected option (-1 if none selected)
• getSelectedValue(): returns the value of the selected option ("" if none selected)

The natualHeight probably needs work. I am not sure how to best size the CBS_SIMPLE combobox. The getTextLineHeight() appears to be different for the selected value box and each of the dropdown items.

Again, I am new to Nim and have never done any Win32 development until last week so I am not sure if everything is done properly but this should hopefully help you roll out this in a future release.