Add optionscontainer abilities

[saroad2]

Added missing abilities in the OptionContainer class:

• Get current tab and current tab index
• Set current tab programmatically
• Get widget from a tab.

Implemented those abilities in WIndows only.

Also added missing unit tests in order to increase coverage in optioncontainer.py. I wanted to get to 100% coverage, but it seems like nobody uses lines 69 and 93. If it's not in use, we can delete those methods.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

As mentioned on Gitter, something about this was grating against me - and I think I've worked out what it is.

It's the _index suffix, and having that index suffix as an attribute that runs in parallel to a current_tab attribute. To me, having 2 attributes for one "concept" (the currently selected tab) is messy.

My preference here would be to have a single current_tab attribute, defined as a descriptor of the "current tab" concept.

This would make the end-user API something like:

# increment tab
myoptioncontainer.current_tab += 1

# decrement tab
myoptioncontainer.current_tab -= 1

# set index of current tab
myoptioncontainer.current_tab = 1

# Get the current tab widget
content = myoptioncontainer.current_tab

# Get the current tab index
index = myoptioncontainer.current_tab.index

# Set index of current tab explicitly;
myoptioncontainer.current_tab.index = 1

# Get current tab, the long way
content = myoptioncontainer.content[myoptioncontainer.current_tab.index]

Going down this path also allows for current_tab to expose a richer API in future. For example, we could allow setting the current tabs by label:

myoptioncontainer.current_tab = "Important"

To be clear - I'm not suggesting this as a requirement for this iteration of the patch; only that it's an option if we make the current tab a rich descriptor, rather than exposing 2 different attributes.

[saroad2]

Fixed the issues presented in the code review 💪

[freakboy3742]

This looks good now, and works well for the Windows case; however it's causing a hard crash on startup with the OptionContainer example on Linux and macOS. It looks like it's a bootstrapping problem with the value of 'current tab' when the app is starting up; I had a quick poke around and couldn't find an obvious fix, though. I'll try to take a closer look later today after I've unpacked a few more boxes :-)

[freakboy3742]

@saroad2 Finally got to the bottom of the problem with Cocoa, and I've fleshed out the GTK implementation as well. If you get a chance to verify that I haven't messed up anything you're relying on, let me know; otherwise I'll let this sit for a day or two, test it again, then merge.

[saroad2]

Hey @freakboy3742 , I checked in Windows and everything seems to be working!
I think we can merge 😃 !