Common widget callbacks
~~~~~~~~~~~~~~~~~~~~~~~

"changed": noarg
	The main user settable propery has changed. For editable input this is the
	content of the widget (e.g. textedit). For tbl_crs*, a cursor position
	or selection change triggers it.

"invoked": noarg
	An option is explicitly invoked by the user. For example enter pressed
	or double clicked on a row in tbl_crs or hand_hovradio or pushbutton;
	left mouse button _release_ over a pushbutton. This is different from
	"changed" because there's no widget state to change but an action to signal
	the user's intention.

"toggled": noarg
	Two-state widget got its state toggled, e.g. checkbox, radio

"cancel": noarg
	The user cancelled currently focused widget operation, e.g. pressed ESC
	on an open menu or clicked away.

"close": noarg
	A non-dialog window of the widget is closed (e.g. in menu)

"keypress-enter": noarg
	Called when the enter key is pressed on a widget where this action wouldn't
	automatically mean "invoked", e.g. single line text edit. It also differs
	from "invoked" that it is always the enter key, never a mouse button or
	other user input that triggers this.

"right-clicked": noarg
	Right mouse click detected. Useful for context popup menu.

"range.moved": noarg
	Called after a change in ranges, e.g. in paned ratio or scrollbar position
	("range current scollbar handle covers").

Special widget callbacks
~~~~~~~~~~~~~~~~~~~~~~~~

"arrow-clicked": widget
	Called from tbl_crs_tree when an tree arrow is clicked. Argument is the
	first column (box) widget in the row, also the row pointer. If the callback
	returns MBTK_EVENT_NOT_HANDLED, miniboxtk runs the default expand/collapse
	logic.