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).

river/PointerMapping.zig

@@ -0,0 +1,25 @@
+// This file is part of river, a dynamic tiling wayland compositor.
+//
+// Copyright 2020 Isaac Freund
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+pub const Action = enum {
+    move,
+    resize,
+};
+
+event_code: u32,
+modifiers: u32,
+action: Action,