config: implement map-pointer command

This command takes a mode, modifiers, button/event name, and pointer
action as arguments. It stores these in the config data structure.

The currently available pointer actions are move-view and resize-view,
which replace the previously hard-coded functionality.

Closing the hovered view with middle click has temorarily been removed
until it is decided if we wish to make this another special pointer
action or perhaps allow running any arbitrary command (which would of
course include close).

doc/riverctl.1.scd

@@ -136,6 +136,23 @@ that tag 1 through 9 are visible.
 	A mapping without modifiers can be created by passing an empty string as
 	the modifiers argument.
 
+*map-pointer* _mode_ _modifiers_ _button_ _action_
+	_mode_ and _modifiers_ are the same as for *map*.
+
+	_button_ is the name of a linux input event code. The most commonly used
+	values are:
+
+	- BTN_LEFT - left mouse button
+	- BTN_RIGHT - right mouse button
+	- BTN_MIDDLE - middle mouse button
+
+	A complete list may be found in _/usr/include/linux/input-event-codes.h_
+
+	_action_ is one of the following values:
+
+	- move-view
+	- resize-view
+
 *outer-padding* _pixels_
 	Set the padding around the edge of the screen to _pixels_.