Let user change `Dropdown`'s style

[ZvikaZ]

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In gui\components\widgets\dropdown.py, line 24:

Currently Dropdown is using a wx.ComboBox with the style hard coded to wx.CB_DROPDOWN. Sometime it's preferable to use wx.CB_READONLY style.

The reason is that with wx.CB_DROPDOWN, when the user clicks the name of the field, it opens up the options to choose from, instead of letting him typing them manually (and requiring clicking the triangle in order to open that chooser), which is less intuitive, in many use cases.

So, it'd be great to let user modify the style. There is a readonly boolean, in _gooey_options, few lines above it, but it seems to be ignored.