checkbox: Adding third state (Indeterminate) support in checkbox

checkbox: Adding third state (Indeterminate) support in checkbox

Summary:
Checkbox should support a third state "indeterminate" along with "Checked" and "Unchecked"
This third state is a state of checkbox which is shown when checkbox is neither Checked nor Unchecked.

- Added this new feature on the basis of a boolean variable's value.
- By default this boolean variable is disabled and checkbox will treat like old way.
- While adding this, I kept in mind, that applications which are already using checkbox, should not be affected, so I used 0=False=Unchecked, 1=True=Checked, and 2=Indeterminate
- Also added an example check_example_o2.c, which is using checkbox with both ways, using boolean, and using enum.
- Now also values can be set using boolean values, but it will give a type casting warning. As a boolean doen't support third state, so I used an enum int like.

- Added APIs to enable disable third state mode. elm_check_three_state_mode_set(check_obj, bool_val), and elm_check_three_state_mode_get(check_obj)
- Modified old APIs which were setting or getting states of checkbox.
- Added a state in theme of checkbox, with third state image.

Reviewers: seoz, raster, Sergeant_Whitespace, Hermet

Subscribers: Hermet, Sergeant_Whitespace, sachin.dev

Differential Revision: https://phab.enlightenment.org/D2249