text: Remove unused requests/events

Put them back in, when we know what arguments they will have.
text: Add commit request

Allows for atomic state changes. Updated surrounding text, content type
and micro focus is taken into account all at once at commit.
text: Add show/hide_input_panel requests

Allows to show/hide the input panel (virtual keyboard) more independent
of focus (some applications might to require additionaly click on a
focused entry to show the input panel).
text: Rename set_micro_focus request

Rename set_micro_focus to set_cursor_rectangle to make more clear, what
it refers to.
text: Split out cursor_position

Add an extra cursor_position, which also allows to change the anchor
(for slections). Change the index type to int to allow setting it before
the beginning of a commited string.

The cursor should not be moved as a direct repsonse to this event but
atomically on the next commit_string event.