Write documentation for sugar3.graphics.ToolButton

[quozl]

Competing pull request replaces #345, #346, #347, and #348, derived from work by @Hrishi1999 and others.

---

Write documentation for sugar3.graphics.ToolButton

• add two examples,

• document critical properties as keyword arguments in the class init,

• ensure documentation is consistent between RadioToolButton and ToolButton, (as some of the text is similar),

• part of feature Sugar3 Docs,

• follow How To Guide.

Derived from work by

• @Hrishi1999 Hrishi hrishipatel99@gmail.com (31176ea) and

• @ndefilippis radicalonion ndefilippis98@gmail.com (db89aca).

[i5o]

Thanks @quozl @hrishi @ndefilippis :)