Tweaks for dealing with display size changes and removal

[mixu]

After using nwm daily for two years, there are a couple of things to improve which crop up occasionally:

• have a explicit reset shortcut, which repositions all windows on the current display and current workspace. Useful if, for example, you remove a display in Virtualbox and want to redo the window positions.
• when the display size changes, recalculate the offsets of all workspaces: right now, only the current one is recalculated which means that occasionally you can see an old workspace until you switch to it
• when hiding windows, reposition them below the current workspace rather than to the left, so that display changes do not cause them to show