Add IsMouseDoubleClicked

[pconerly]

Adding this binding. (Especially needed since pyimgui has imgui.SELECTABLE_ALLOW_DOUBLE_CLICK available!)

I went to update cimgui.pxd but IsMouseDoubleClicked already had a check next to it. Are the check/x marks in cimgui.pxd out of date? Or do they not reference the pyimgui API?

Great project btw, @swistakm

[coveralls]

Coverage increased (+0.01%) to 41.503% when pulling 6ed8f5e on pconerly:is-mouse-double-clicked into 6486f85 on swistakm:master.

[swistakm]

The ✓/✗ are usually up to date but sometimes it happens that something slips through.
Their main purpose was tracking overall progress but I have started using nm and c++filt for that (see: ci/completion.py script). I will definitely get rid of them completely some day but for now they are sometimes useful because I often switch between macos/linux/windows boxes and doesn't always have the all the tools and this is then just useful reference. I see now that some other IsMouse* are also incorrectly marked so I'll fix that.

Anyway, this PR is perfect. Thanks!

[swistakm]

Released as 0.1.0